RoYIDG

You all need to have the exact same mods, but prepare for a buggy ride. And ALOT of CTD's. To reduce crashes, remove all mods except LCPDFR. 

Yup, the IVdashcam will lag your game for sure.  Had to remove it as well, now I'm running a much less demanding radar script (PSS I think).
 
DrDetroit

Okay, this happened to me, if you have IV Dash Cam, TAKE IT OFF. I went from 15 fps to running 40. For some reason, my computer lagged because of IVDashcam. If you have it installed, I'd try that.

GTAIV is not designed to use multiple cores and it will only use one. It depends on how fast each core is in your processor. Also it may be your 2 gigs of ram that is causing this.

In game, turn off shadows completely and set water quality to low.  Everything should be low/med.  Check graphics card settings; reduce mipmap to performance, and turn off (or set to Application Preference) AA and AF.  Turn off all unnecessary application that are running in your Task Manager processes and services.  
 
Also, you need more RAM, pickup 8-16 gigs if supported by your OS.  RAM is pretty cheap right now.
 
Good luck,
 
DrDetroit

Hi, I have the same graphics card as you and I get around 25-30 FPS. It could something to do with your RAM, which is 2GB (mine is 8GB). It could also have something to do with your processor since it's a dual core and only 2.5GHz (mine is a Intel Core 2 Quad Q6600 @ 2.4GHz (Quad Core)). To fix the frame issues you could try upgrading some parts in your PC, such as the processor and RAM (be careful, you have to get the correct frequency of RAM for your motherboard).