Ok, this PC has almost reached it's EOL (end of life). For a very casual gaming rig it might work, but you should invest a maximum of 100 $, and not a penny more! Being a BTX-case, upgrading the graphics card already qualifies as a separate topic.
Anyways, here is my suggestion: Core2Duo E6700 (don't confuse it with DualCore E6700, which won't work!), e.g. http://www.ebay.com/itm/Intel-Core-2-Duo-E6700-SL9S7-SL9ZF-LGA-775-Socket-2-66GHz-1066MHz-CPU-Processor-/311599279383?hash=item488cc3ad17:g:Z-EAAOSwQSBXHSif
AMD Radeon HD7750 Single-Slot (Dual-Slot cards will not fit!), e.g. http://www.ebay.com/itm/HIS-Radeon-HD7750-H775F1GD-1GB-128Bit-GDDR5-PCIE-Video-Card/162045823654?_trksid=p2047675.c100010.m2109&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D35389%26meid%3D88186d715d6948e19cbe83d1c1322ebd%26pid%3D100010%26rk%3D1%26rkt%3D7%26sd%3D291746426239 (but keep looking for new deals with cheaper prices!)
4-8 GB Ram DDR2-PC5300, e.g. http://www.ebay.com/itm/4GB-2x-2GB-DDR2-667MHz-PC2-5300-DESKTOP-Memory-RAM-Non-ECC-667-Low-Density-SDRAM-/331688948163?hash=item4d3a33b1c3:g:VXIAAOxyoahSVLW4