I did a similar thing recently - picked one up dirt cheap, so far been enjoying PGR2 the most. Its the perfect balacne of semi-realistic racing for a console IMO. Looks good too.
Forza I'm undecided on - its a fun game and more realistic in many ways than GT4 - I'd say its them most realistic console sim except perhaps Enthusia, but has a crappy tyre heat implimentation, runs at a lower framerate than most racing games (locked at 30fps or so?), some of the tracks like silverstone have a pretty overdone fog affect so you can't see the next corner coming up, and the Xbox controller isn't very good for it. Had to switch to using both analog sticks PS2 style to get anywhere. Forza's version of the 'ring looks great though.
Burnout Revenge is a brilliant title IMO, its not semi-sim or anywhere near it but its great fun in a completely different way. The multiplayer works very well too.
Also picked up Juiced & CM2005, both suffer from crappy gameplay, and cars have the floating feeling.