Prairie du Chien or Fond du Lac
cities in Wisconsin
as Ace Ventura sez
Jim Carrey played the whacky character Ace Ventura in two movies (1994 and 1995). His signature catch phrase was "allllll righty then."
Kum & Go
A chain of convenience stores popular in the Midwest. Hard to believe Pynchon didn't make up that name!
According to WIKI, the name is "a play on the phrase "come and go" using the initials of founders Krause and Gentle."
Floyd's Knobs, Indiana
A real city with a Pynchonesque name.
Merc ... CME
Chicago Mercantile Exchange. Although New York City is the center of stock and bond trading in the U.S., the main locus of futures and options trading is in Chicago, with activity split between the CME and the Chicago Board of Trade (CBOT).
Double trading is an options strategy that experienced traders use to (hopefully) double their profits. After they have purchased a contract, if traders notice that price of the asset is on a continued upward (or downward) trend, then they will buy more of the asset. (At least that's the only definition I could find. I can't imagine why the strategy would be banned, and couldn't find any instance of such a ban ever taking place, at CME or anywhere else.)
- The passage makes a much better sense if one assumes that by "double trading" Pynchon actually means "dual trading", i.e. the practice whereby futures traders trade for customers as well as for themselves at more or less the same time, which was banned at CME, for high-volume futures contracts executed on the same day, in May 1991 (see CME Rule 552).
...They sing her the Hy-Vee commercial. More than once.
Appears Horst was successful in giving the boys a crash Iowa-childhood experience.
at the CBOT, and to the Brokers Inn, where they ate the legendary giant fish sandwich
This is all true....
- Years ago, before the new 10,000 CBOT floor was open, and the CBOT was only one building, there used to be a restaurant in the building just to the east. It was called Broker's Inn. [...] As a tradition, on Ash Wednesday and every Friday, Broker's Inn would serve "the fish sandwich". The fish sandwich was a large yellow bun, served open-faced, with fried whitefish piled as high as the plate could hold before falling off. The fish was light, tender on the inside, and crisp on the outside. This was served with their famous tartar sauce and fresh wedges of lemon. One person could never finish this sandwich if they ordered anything else. This was all before the new floor was installed. 
Here's a recipe for a Maid-Rite.
Louisville Hot Brown
Wow, WIKI has a page on this sandwich.
Founder of Atari and Chuck E. Cheese.
Another Scooby reference to go with chapter 18's "Daphne and Wilma's."
As with the other arcade games mentioned in the chapter, Hydro Thunder really exists. Wiki
covering a margin call
Horst has purchased some derivatives (options or futures) with a significant fraction of the purchase price loaned to him by his broker ("purchased on margin"). Since the time of purchase, the price of the derivative has gone down, and the broker has given Horst a "margin call", ie a couple of hours to put up some more cash to limit their level of exposure. If Horst fails to do so, and the price keeps dropping, the broker stands to lose money. To prevent this possibility, the broker will just sell the derivatives at the going price, which would generate an instant loss for Horst. In this case, Horst believes the price drop is just temporary, so he deposits the extra cash as requested ("covers the call") and the derivatives don't get sold. It looks like he has a number of other positions open, so this margin call is not necessarily a big deal. It does indicate that Horst is engaging in high risk trading though: leveraging his capital in a market that is already very volatile and risky.
Berghoff is also a German restaurant on West Adams near the financial district. In the window, Berghoff displays Chicago liquor license #1.
Rachel . . . Green
Rachel Green is the character played by Jennifer Aniston on the TV show Friends. She had an on-again, off-again love affair with Ross.
ha ha only serious
A phrase from "nerd"-ier circles and mentioned in the Jargon File:
[from SF fandom, orig. as mutation of HHOK, ‘Ha Ha Only Kidding’] A phrase (often seen abbreviated as HHOS) that aptly captures the flavor of much hacker discourse. Applied especially to parodies, absurdities, and ironic jokes that are both intended and perceived to contain a possibly disquieting amount of truth, or truths that are constructed on in-joke and self-parody. This lexicon contains many examples of ha-ha-only-serious in both form and content. Indeed, the entirety of hacker culture is often perceived as ha-ha-only-serious by hackers themselves; to take it either too lightly or too seriously marks a person as an outsider, a wannabee, or in larval stage. For further enlightenment on this subject, consult any Zen master.
Could be an abbreviated form of "network effect," where a good or service becomes more valuable when more people use it, which is the case for many websites such as YouTube, Amazon.com, Wikipedia &c &c.
There's also "Twork It Out," a tune on Usher's third studio album, 8701 which was originally intended for an October 31, 2000 release but was delayed numerous times following the leak of several tracks onto the online music store Napster and finally released in July 2001. The title is derived from Usher singing for the first time in his local church in 1987 and the album's release date of 2001. From the lyrics, "tworking" is pretty much having sex.
End of the World As We Know It
A reference to the song "It's the End of the World as We Know It" (1987) by R.E.M.
As Labor Day approaches
which, in 2001, was on Sept. 3rd
"Atildado" is Spanish for elegant, stylish
Or more pointedly, it means, The Dapper Man.
you mean 'Dude'?
Johnny Pacheco is a Dominican musician, arranger, producer, and bandleader of Cuban music. He is one of the most influential figures in Latin music, best known for being the creator of the Fania All-Stars, and for coining the term "Salsa" to denote the genre. From WIKI.
A barstool, named Sven?
Maybe a bit of poetic license here? IKEA has a stool named "Svenerik", but it's more of a piano stool. The max height is 58 cm, so it's too short to be much good as a bar stool.
Jemima, Keziah, and Kerenhappuch
Job's three daughters, as in, from the Book of Job.
Merengue is a style of Dominican music and dance. Partners hold each other in a closed position. The leader holds the follower's waist with the leader's right hand, while holding the follower's right hand with the leader's left hand at the follower's eye level. Partners bend their knees slightly left and right, thus making the hips move left and right. The hips of the leader and follower move in the same direction throughout the song. Partners may walk sideways or circle each other, in small steps. They can switch to an open position and do separate turns without letting go each other's hands or releasing one hand. During these turns they may twist and tie their handhold into intricate pretzels. Other choreographies are possible. From WIKI.
Classic song by Mr. Barry Manilow.