I need help with something that has been rather difficult to troubleshoot.

I had been getting artifacts on the screen, FREEZING, and blue screens at first.  After several crashes like this, it began to freeze almost immediately after start-up.  I could prolong the freezing in safe mode, but I was SURE it was a problem with my video card and swapped it out.  I took out my 8800 GTS that I had been using for almost a year, and put in my old 7600 GT that had still worked the last time I used it.

After the switch, some problems disappeared and some remained.  The constant freezing stopped, but artifacts were still there; just different kinds.  Instead of colored lines, I now had dots all over that flickered and FADED.  They weren't as distracting as the lines, and playing video and the internet still worked fine, so I stuck with the old card for the last month or so.

The big problem that REMAINS, is when I try to run any 3d based game.  (old 2d games still work)  The games seem to load without a problem, but after a MINUTE or so of playing, there are polygons that "stretch" out and cover the screen, as well as repeated screen hitching every few seconds.

I just find it really strange to have 2 very similar sets of problems with 2 different video cards, but I don't have a second computer to test on, or the experience to pinpoint the root of it.  Am I looking at something like memory failure, a dying motherboard, or power supply problems?   Or, as unlikely as it seems, are both my video cards done for?  I'm just looking for some solid advice so I don't end up replacing anything that is still good.
Thanks for reading this, any help will be appreciated.When you swapped the 7600 in there, did you reinstall the driver?Yes, I've reinstalled the drivers several times for both cards, thinking I may have had the wrong one, but I'm pretty sure they're the correct ones and up to date nowwhat are the specifications for your computer? it could be other hardware affecting your cards performances: like lack of power from the PSU or a faulty PCI-E x16 slot