20 grand is more than enough for a nice, clean Camaro or Firebird. Pick up an LS1 based car, add gears, headers, exhaust and you're good to go. An LT4 based C4 Corvette is also a pretty sweet deal right now.
20 grand will also get you into an older M3 or S4. Maintanence will run you a bit more, but you get a lot of car for your money. A '95 M3 with intake/cams/ECU/exhaust is a fine running machine. A chipped S4 is a monster ride... provided you take care of the motor.
20 grand on something Japanese... I'd go with an S2000. You should be able to find one if you look around. You can also try a real Type R Integra, but they are pretty hard to come by and there are a million wannabe fakes. I'd definately stay away from some of the ragged out modified Tegs you see crawling all over... unless you know it personally.
Well.. that's off the top of my head. I've had a few glasses so I probably forgot something good...
EDIT: Christ...you gotta be kiddin me. Friggin F-Body again.
M