Welcome to the club of 7900GT problems. I had my EVGA 7900GT CO Superclocked (550/1550)for about 6-7 months also.
I praised the card to all who would listen, until now. I used to play EQ2 for 5-6 hours daily and haven't been playing much lately. LoL.
Do a little searching and you will find an inherent PROBLEM with the 7900GT's that no one want's to fess up to. They CLAIM their problems are fixed, however are still RMA replacing w/ what I call "try and see" card. I RMA'd my card. After a 3 week wait I got one back that was actually factory UNDERCLOCKED. It worked for less than a day. On the second day it went back to artifacting, only now worse than the original card. Also caused my PC to lock and blink screens (Read about it, every 7900GT that goes has these SAME problem) Rebooting it makes it ok in normal 2d windows tho. And YES the problem WILL get worse over time so don't second guess it. Get your RMA going ASAP.
ANYHOW, they will try to convince you it's the PSU. It's ANYTHING but their card they will say. However, you do some googling and you will see... These cards are FLAWED! Hence the 7950's release, but they will never admit it.
I read forum posts where people have RMA'd there 7900 as many as 6 times! Also read where one poor guy bought 3 PSU's b/c they convinced him every time he bought one it wasn't good enough, tho it met specs. ROFL!!!
All I got to say aobut the 7900 is Good Luck Bro
The 7900 problem seems to be more a Nvidia problem than the manufact of the card as ALL manufacturers are experiancing this. My next card will be a Radeon for sure. They just sign sealed and delivered my loyalty to Radeon by these swap'n see's they keep doing.
This shout goes out to Nvidia - keep playing your games with me and not giving me a working card. I won't be playing my games with you anymore! I'll take my ball and go home! (Figure of speech)
My opinion; if you have a 7900GT and a problem... you should get uped to the 7950GT no questions asked instead of playing the swap game for months on end.
That's just my 2---