Yeah, Forza2 is getting on in age now, but the gameplay is great, and you can download vehicle updates from X-Box Live, which is how I got the S5 in the game (March '08 vehicle pack). The R8 is in the December pack I believe.
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Some observations: my Supra buddies and I all like to dial in our actual suspension settings used on the road (camber, toe, spring rate, etc) and the game has been surprisingly accurate in terms of simulating what the car actually drives like. If the S5 in the game is equally accurate, then the car does actually tend to oversteer when you toss it into a corner, which you can cancel out with gradual throttle. The RS4 in the game does not exhibit this behavior; it's more like a heavy Subaru (mostly wants to go straight until you really tweak the suspension), whereas the S5 actually feels more like a heavy Gallardo. It's also worth noting that you can swap in the RS4 engine or the old 2.7TT to see what that's like. =P
</geek notes>
-Ray