Hopefully someone can help me with this maddening problem. I've got the following matx system setup:
Asus M2NPV-VM Socket AM2
Athlon 64 x2 brisbane 3600+ (stock hsf, no overclocking)
WD 200GB 7200RPM SATA HD
OCZ PC2 5300 2GB (2x1GB) RAM
eVGA Geforce 7900GS KO PCI-E 16x
Apevia X-Qpack 2 mATX case with Apevia 500W PSU (dual 12V rails)
I've been having problems straight from the get-go with the 7900gs display intermittingly going blank (no output when computer boots), and also the display (not the computer itself) "freezing in place" at random. I was able to play FEAR and Oblivion other times with no apparent problems, but the card just stopped working altogether one day (the onboard video still works). So I did an RMA and received a (new) 7900gs, but a month later the same problem has rendered it non-functioning again. Could the problem component be the PSU? It seems generic but has fared pretty well review-wise on newegg (http://www.newegg.com/Product/Product.aspx?Item=N82E168...), so I'm really not quite sure wherein the problem lies. What else could be causing this problem? Can the PCI-E slot be defective or is it something else involving a different component?
Could be the motherboard or PSU. Could also be integrated graphics... did you disable them before popping in the new card? VGA cards and on-board graphics generally don't get along very well. *edit* You can also try a driver cleaner to make sure the old drivers are completely gone from the system before using the new card's drivers.
If it's the motherboard - it's very hard to troubleshoot with a duplicate board right there by your side.
If it's the PSU - Do you have the additional 6-pin power adapter plugged in? Is the VGA located on it's own rail? (ie. Is your PSU connected directly to your card or is there a fan, HDD, etc. plugged in on the same rail as the VGA?) Try isolating the card to it's own rail or even try a different rail to see if that helps any.
That may well be your problem. Apevia, formerly known as Aspire, is a tier 5 psu. Its not recommended for anything, unless you need a target at a rifle range. I went through this a few years ago when I bought an Aspire 680wt psu to power a SLi rig. There was immediately power problems. Research turned up that the 680wt rating was peak power, with a continuous rating or 380wts, far below what I needed. No telling what your Apivia 500wt really puts out. with a lack of power, your 7900 card may either not work at all, or will freeze up once the power requirement goes up.
For a psu tier list, look in the forums under power supplies and a sticky by Jake Barnes. At the bottom of the page will be a list of several links, including one for psu tier ratings. I'd advise looking for a tier 2 psu, unless you can afford a tier 1 psu.