well this is odd.
So I get that X1900XT card, try it in the computer and ....no signal , nothing, not even distortion , just no signal at all , the monitor was on standby waiting for the signal . I could hear the Windows Welcome tune which means its booting, but there was no signal (taking out the CMOS battery didn't do much either).
Eventually I got sick of this and decided to test it on a modern machine , and it worked the first time, no problems.
I decided to try that 7950 GX2 card in there (obviously this isn't on the compatibility list, given its a machine from a year ago) , and what do you know, it works first time too , no funky corruption at bootup, no crazy glitches, it just booted into Win7. I didn't install the drivers. but 2 minutes later when I put it into that ASUS A8N-SLI board, the corrupted visuals came back.....
back to square one I guess.... so it may be a problem with either the motherboard, or PSU Im guessing? the PSU a 550w PSU , so in theory it should be fine....