You really need to look into getting more RAM... really the minimum you should have to play any game is 4GB, and a lot of games end up using more than 4GB even, and maybe a 64-Bit OS would be of benefit as well...
Go for a 750 or 750Ti if one's available. The 750 is equivalent to the 560, and is stronger than the 650, but the 750Ti is stronger than both of them.
The 750 is 1GB, and the 750Ti is 2GB.
get what you're budget is aiming at i guess it's pretty low considering you wanna buy a gt630 so get a gtx650 i think that's lowest gtx card that is still selling preferably the 2gb version now about the processor and the ram you wanna run gta iv and gta iv is not optimized for the pc that much and requires a multi-core cpu the graphics card isn't that much required as the cpu in that but i think you will be fine playing it at 30 fps on medium, now about the ram 2gb is not gonna be enough if you plan to play games later than about 2010-2011
Your post is very hard to read. Please punctuate it properly; or write in multiple paragraphs.
The 650 cannot use 2 GB of VRAM. I have run some pretty demanding games on my own 650, and I didn't hit a VRAM wall once(including GTAIV where it tops on 960 MB used; that game just consumes and consumes, you shouldn't base your VRAM needs on that game).