I vote for NOTHING based on your current system:
Ga-P35-D3SL, E6750(Oc'ed to 3.4Ghz) w/ Arctic Freezer 7 Pro, 4 GB G.Skill (DDR2800, Linked), EVGA 8800gt 512 MB, Corsair VX 550w, WD 160GB Hard Drive, 1TB Fantom Ext. HD, Antec 900
If you feel the itch to upgrade, just swap out your video card.