The one that hasn't been suggested here is a Chevy Cobalt SS supercharged. At just shy of $22 000 MSRP new, it's a bit higher than you said you wanted to spend, but with all the dealer rebates going around right now you might be able to get one a bit cheaper. They look decent stock, you'll be able to improve on it easily enough and with 205 stock horses it oughta be quick enough. It also comes stock with 18" aluminum wheels and Z rated tires, so that's one less thing to worry about if you plan on making trips to the drag strip. The only possible downside is it only comes with a standard transmission, so if you'd rather let your left foot go to sleep you're out of luck.
Cobalt SS :
The Saturn Ion Redline has been mentioned, they're pretty sporty looking and nice enough cars, but that gauge pod in the center of the dash would just drive me nuts personally. The Ion seems to be selling well enough, so I guess not too many people have a problem with it.
Ion Redline :
And finally, you could get a beater for a year and save up some cash for a Pontiac Solstice or Saturn Sky (same car, different nameplate). They're nice little sporty roadsters and while not as powerful as some of the other cars mentioned, 170hp is almost what I make and my car weighs a good 500 lbs more than these things. It'll get along. That and with an Ecotec under the hood getting more power out of it really isn't all that hard.