My dad recently bought the 8500 for his PC(since blowing his old one ¬_¬) And it performs really well, especially as it was pretty cheap. Now, I recently read a review comparing the 8600 with ATI's equivalent, after quite a lot of benchmarking etc on different games, in both DX10 and 9 environments, the nVidia beat it hands down.
I have a 8800 GTS which is awesome, I'd recommend it if you've got the money, or go for something cheaper like the 8600, but I'd definitely stick to nVidia