Dude, i feel your pain. I got a Asus 7900GT.
I get artifacts on the BIOS screen and on the Windows XP boot screen. Watching movies, i get dots all over movie screen. Played Crysis, NFS, Witcher, all gives stretched sprites and lots of tearing and other wierds images.
With the help of google, the 7900 series was a flop. Most people only starting experiencing the problem after +/- 8 months. Nvidia knew they were launching the 8 series so they never bothered to fix the issue.
Most people just RMA the card. After searching hundreds of sites for another solution. I came accross a thread where the guys says part of it is due to overheating. If you look at your card underside (the side that faces the ceiling), in the top right corner there are two IC's...these two IC's need to be kept cool.
I dismantled my fans from the side and back, placed on the graphics card so it would hit the IC headon. After an hour, no artifacts, nothing on BIOS and windows boot.
Removed and replaced the fans 5 times, with my friend as a witness and when kept cool no artifacts.
I know its not the best solution and sometimes i feel robbed cause of paying like R3000 when it came out.
Try googling, there was even youtube vid with guys experiencing what we getting...
Hope this helps