Upgrading my CPU or GPU?

[KvothetheArcane]

Hello all! With X-Mas just around the corner I've found myself in line for a nice little bonus at the end of the month, and am wanting to upgrade either my CPU or GPU? I've noticed my system is lagging a bit in games such as Skyrim when I turn on some mods, as well as Battlefield 3 and Shogun 2. Here's my current set-up:
Mother Board: MSI P67A-G45
GPU: Nvidia GeForce GTX 560
CPU: Intel i5-2400 3.4GHz
6144mb RAM
Windows 7 32 bit

Thanks! ^^

EDIT: I guess I should state what games I'm looking to play; Far Cry 3 (Ultra or Very High), Battlefield 3 (Ultra), Guild Wars 2 (Ultra), Planetside 2 (Ultra), and maybe some Crysis 3 (I'd like to hit at least Very High)

[Zadoc]

32 bit Windows with 6GB of memory? Your OS will only be seeing approximately half of that (3.5GB or so), I would consider upgrading to a 64 bit flavour of OS. WoW will certainly use the extra, not sure about the rest.