only a 6600gt is enuf to play the game on full settings , no need of buying 2 in sli (but better is always good if u have planned that much to spend)
and i recommend u buy atleast 1gb RAM , 512 ram will bottleneck the nfsmw(and other games too) performance . although it is quite well playable on 512 ram , but u don't want rare loading stutters in quite big races in nfsmw after u have spent that much on your rig.