Well the graphics are pretty much the same you get with all at max , but I took the GTX 8800 becouse :

A I was going to buy an new PC
B I work with 3d stuff and I wanted something that coulfdhandle my renders fast
C I wanted a direct X 10 card
D I assembled it myself so I saved some money
E I have a high res Monitor and it needs a very good graphic GPU ...
D Dunno what else but this card is great ...

at any other setting inferior than the ones I posted the game runs smoothly and fast ...

On my other pc wich has a ati 9800 mobile with 256mb ram the game runs fine if I deactivate shaders 2 and actually looks running a bit better than RTW itself ...

Btw you notivced that on huge settings we no more have 200 men units?