Unless you wanna work to build a dash there aren't any nice kits for a 1998 Grand Prix. My favorite nice deck though has got to be the DEH-P800PRS, it's got a great EQ, 5volt pre-amps and I'm sure you can pick it up a lot less than 550 if you check ebay, or sonicelectronix.
http://www.pioneerelectronics.com/PUSA/Pro...ier/DEH-P800PRS
EDIT: Just read the 200 or less I say 5100, or wait a few months till the 6100 comes out and try to pick it up in your price range.