What's the stock speed?
You can voltmod graphics cards, either via the GPU BIOS, or by using conductive ink on the card, to give the core/memory more voltage. I voltmodded my 7900GS, and got another 80-100Mhz out of it. Changing the voltages on graphics cards requires more work than changing the CPU voltage, just because of the nature of the work being done.
So if your artifacting, your at your max. You can either stop, or you can do some research and voltmod the card if you wish.
RAIDMAX SMILODON Case
LANPARTY DK P35-T2RS
Intel Core2Duo E6600 (2.40) @ OC'ing
2 gigs G.Skill DDR2 800 RAM @ OC'ing
2x 320 Seagate Barracuda 7200.10 (RAID 0)
EVGA Ge-Force 7900 GS KO @ 710/985 @ 1.48v/2.20v
LITE-ON 20x DVD+/-R Burner/12x DVD-RAM w/ LightScribe
LITE-ON 48x CD Burner/16x DVD-ROM
Hanns·G 19" Widescreen LCD
Windows Vista Ultimate Edition