Processors, Graphics Cards, and Gaming?

I play games Like All Points Bulletin (APB), WoW, and stuff simple like Minecraft. My birthdays coming next week and I can upgrade 1 part to my PC. In games like APB i lag like crazy and my PC is all the way at 100% CPU. Games like Minecraft only lag online and sometimes have the smallest bit of lag when I play. Is the problem my NVIDIA GeForce 7300 GS Video card or my Intel(R) Celeron(R) CPU 450 @ 2.20GHz, ~2.2GHz Processor?