I don't have much spending money so I need to upgrade my computer piece by piece overtime. I'm also not that knowledgeable about whats good or not, so my biggest question is this. My computer is struggling with playing games right now (usually does the worst when lots of people are on screen in like WoW or marvel heroes), what should I upgrade first to try and make this better.

Here is my current set up:

GPU: GeForce GTX 560
CPU: AMD Athlon II X4 640 processor
12 GB ram DDR3
motherboard: MSI NF750-G55 (MS-7578) (CPU1)
Hard drive: 1863 GB Western digital WDC WD20EZRX-00DV080 ATA Device (SATA)

Thanks so much for the help.