GW does not have very high demands on gfx cards. I use a 6800 ultra - an ancient card by computer standards - and it runs GW at 1600x1200 and all graphics maxed (anti-aliasing, anisotropic filtering, effects - you name it) without framerate ever dropping under 40.
Your 7900 GT is actually considerably better & faster graphics card than mine; your card is probably good enough to play Guild Wars 2 when that is released, never mind the present Guild Wars.
Before you start replacing what is a perfectly good-enough card, I'd suggest checking two things:
1) Overheating of the graphics card gives exactly the kind of artefacts you describe.
Do the artefacts appear instantly, or does it take a while of gaming - minutes or hours - before you start seeing strange effects?
If there is a delay, that strongly suggests overheating.
IF the card is overheating, it may be due to, for instance, dust build up on the fans, or maybe a fan has completely stopped working.
Also, if you overclock your graphics card (or use drivers which overclock them by default) that is likely to lead to overheating. Lowering the clock might then help.
2) Graphics card driver. Download the most recent driver for Geforce 7900 GT from
http://www.nvidia.com (it doesn't matter what brand your card is, the nvidia driver will work; manufacturers don't write their own drivers, but just take the nvidia driver and slap on some usually useless fluff for their cards). If you're really lucky your present driver is buggy or corrupt, and installing the newest driver may then help.