Ah, it looks like GTA IV is the first game ever to be written to take full advantage of the quad core architecture. So that's at least 1 game that can actually use all 4 cores, heh.
While prices are dropping all the time for computer parts, I think there is still a noticeable price gap between quad and dual cores at this point in time, which is especially noticeable with the Intel chips, over $150 difference between comparable dual and quad cores at similar speeds (E8400 is $165 on newegg, Q9550 is $320). And unless the market shifts significantly over to quad cores, there will still be dual cores being made and obviously those will be quite a bit cheaper than the quads or otherwise there would be no point in buying dual cores if the two prices are too similar.
The question is whether or not there is significant and noticeable increase in performance during game play. Peptiko mentioned he is looking for a mid-level gaming rig, so unless a quad will give you 10+ FPS increase on average (bringin choppy ~20FPS up to a playable ~30FPS) I'm guessing the performance won't be noticed unless all the other parts of the system are decked out as well, namely the video card that hand support the graphics. But of course I'm not sure if there have been any benchmarks out comparing dual and quad core performance for GTA IV, but it'd be interesting to see just how how much of an improvement the quad would have over the dual and then try to decide whether it is worth the extra money.
But for pepitko, who is looking for a mid-level gaming rig, I think dual-core is good enough. Obviously, it's his money and his choice, and if he can find a cheap quad then all the power to him. Unless, of course, dual cores are being phased out and every single game from here on out being released will perform significantly better on quads than duals.
edit: eh, may have to start a new thread instead of hi jacking this one