something is wrong there.. I can play NFSU2 on medium settings fine on a Geforce 6200 256MB (not turbocache), 512MB RAM, and a Sempron 2800+
the only thing I think may be worse than my PC is the Celeron. they aren't exactly designed for gaming.
I mean, I know my Sempron isn't extremely powerful, but I think having the AMD K8 architecture makes it better for gaming
but you could always try some things like disabling Windows services, turning off paging file, installing new drivers etc...