What graphics setup are you running? I realise it's onboard from your post (as you state you don't have a graphics card) but what kind of motherboard are you running that is producing your graphics?
First impression, no you won't be able to play GTA4 on on-board graphics. Although it's very heavily CPU dependant, it does still need some (fairly) heavy input from the Graphics Card so I would've said you WILL need one. That said, you say Far Cry 2 runs on your system? What resolution and graphics settings are running FC2 on, and what settings do you hope to run GTA4 on?
Sorry for all the questions, but give us a little more info and someone'll come up with a definite yes or no, I'm sure.
@ Welshmousepl - There must be something wrong with your setup then dude. I have a Phenom I (fixed TLB) 9950 2.6 GHz and GTA IV runs perfectly smoothly in 1900 x 1200 with med-hi settings!
@balusachin Your CPU should cut it OK, but you will need to buy a discreet graphics card, I don't know of any integrated solution that can play anything more than solitaire! If you can indeed play Far Cry 2, then I would suggest that you perhaps you do have a discreet graphics card and don't realise it.