I would tell you to get a new computer since your mobo doesn't have an AGP port. A second-hand Pentium D system can be had for around $300 on eBay which would be a much better base for a gaming system. But if you don't have enough money you can get some more RAM. Your system supports PC3200 DDR RAM according to Crucial:
http://www.crucial.com/store/listpar...02400%20Series
And maybe get a PCI card like a Radeon HD2400 Pro:
http://www.newegg.com/Product/Produc...82E16814131082
Do note that you might need to upgrade the PSU capable of 18A on the +12V rail to run the 2400 Pro. Even then you will not be able to play very intensive games like Crysis. If you really want, you can also upgrade to a motherboard that has an AGP slot, just make sure it suits your CPU and fits in your case.