If you want a realistic (aka sim) game, then you'll find that PC sims are far superior in terms of physics than Forza 2.
However they do generally lack content when compared to things like Forza (both number of cars and ability to modify the cars).
But, some to look at:
Live For Speed - Got a demo, and full game is only £24, not bad but never wow'ed me
rFactor - Again demo available, although standard content sucks, massive mod market though
GTR/GTR Evo/Race'07 - Basically the 'SimBin' games, all quite good with a wide variety of cars across the different games
Richard Burns Rally - Old now, but still the best Rally sim around
Grand Prix Legends - Another old one, but imo it's one of the best out there, I'd actually rank it 2nd to:
iRacing - not flawless by any means, cost being the biggest flaw (it's a monthly charge + content as well). But it is also, imo, head and shoulders above the other PC sims, in terms of graphics, physics and best of all the online Licence/Safety Rating/Championship terms.