GFX upgrade, GTA Vice City stutters?

Hi there, I need some guidance here..
GTA: Vice City runs better on my old system (Pally 1700+, 256Mb PC 3200, GF2 MMX400) than on my new and faster sytem, (2800+ @ 3200Mhz, 512Mb PC 3700, GF FX 5900 (Creative Blaster 5). with the latest drivers, etc...

Why should this be? The main problems are sluggish graphics, especially when exiting a car?
and occasionally while just driving
it just gets me down, when GTA Vice City should run much better with a more powerful GFX card than a GF 2 ! Vice City is optimised by DX9 so it says in the installation Read Me files.
Are there any other settings I may have overlooked?
thanks