1) get that 80 pin ide cable and replace (this is the connector that goes from Mobo to HDD) not sure if it'll help much with the stuttering but would definitely improve your PC performance in general
once replaced, you can verify transfer mode (UDMA5) from device manager
2) try using stable drivers like 84.xx series fro your 6600GT
3) Get another 512 stick as Saumil suggested if you can
NFS MW works perfect on my Go7400 system, no reason it should stutter on your 6600GT...perhaps 1Gig of memory on my system helps..haven't tried with 512..