Jalopnik put up a cool article about the real life cars that inspire the cars you would find in Grand Theft Auto 4. While not being comprehensive, they definitely hit the nail on the head for the faster cars you would find in the game.
Sultan RS
This one goes out to the tuner crowd that is definitely playing this game. Seriously, we can’t even enjoy a video game without carbon fiber madness? The headlights and spoiler say Lexus IS/first-gen Integra. In terms of performance, the Sultan RS is pretty speedy at 186 mph and the handling is nice with the AWD and decent brakes.
One thing they forgot to mention was that this bad boy has a very nice blowoff valve to go along with all upshifts. There are VERY few of them in the game so if you find one, hold on to it. Check out the full article to get information on the rest of the cars.
Auto-Inspiration: The Real-Life Cars Behind Grand Theft Auto IV