GTA4 requires a few things to run smoothly. Lots of CPU power (Quad preferred to Duo), lots of GPU Memory, and proper settings: clean and fresh latest Nvidia drivers + the latest game patch(es).
The more memory you load onto your settings (even if still green), the more CPU power is required (i.e. traffic). Although your new GPU may process some of the display settings better, the overall settings becomes harder on your CPU, which is most likely your bottleneck.
That said, if you say it ran great before, try the same settings as before and troubleshoot from there (incl resolution).
Nota: Your new GPU processes graphics completely differently than the previous Nvidia GPU family. My guess is you need either a game patch or you need to download a driver cleaner and re/install the latest nvidia drivers.
Hope this helps.
Hey, thanks for your reply's
I installed the latest NVidia Drivers, and it worked! the game RAN smoothly
But when i came back and turned my PC on, again 1 fps