For GP4 only, Tom might be right. An 8800 GTS is definitely good enough for GP4 in full detail, but the question is if the graphics shall really only be good enough for that. A quick look at Amazon.com shows that a new 8800 GTS still costs around 110$. And a used one 60-80 $ (where you never now how the hardware was treated before and how long it lasts).
For 130-140$ you get a new GTX 460 1 GB graphics card, which I use myself. It is a high middle class card of the second to newest generation and is very energy saving. I would, at least, consider spending 30-40$ more to have a new and very decent graphics card.
GPGSL : Team Owner of 'Maverick Track Performance' (MTP)