AN I5-2500k or a I5 2400 is the way to go if you want to spend that much. A AMD PhenomII x4 965 is another option if you want to overclock to 4.0-4.2ghz but you'll need a cooler. $25 gets you a decent cooler for an I5 or PhenomII.
Coming from someone with a 4.5ghz phenom 2 x6 1100t(the best amd processor out), Go with the I5 hands down more power for gaming, I myself find im constantly being bottlenecked by high end games like witcher 2, crysis 2 etc i dont even hit 70% load on my gtx 570 when vsynch is off lol. This has me ordering an LGA 2011 I7 3820
even though the i3 or i5 is not quad core for games like gta 4
afaik mobile sb i3 and i5 are dual cores. and only one desktop sb i5 has 2 cores.
gta4 is a poorly coded console port that can tax even the best configured pc. it likes cpu cores and ram. if you're going to play gta4 on a laptop, sb i5 with strong discreet gpu or amd a8 + dual graphics. you might have to lower some gfx settings.
i should add that, trinity apus are on the way and they promise to deliver better gpu performance than current llano gpus.