System freezes or screen goes blank

I used to have the radeon 9550 vivo version, which was fine, until one day I could no longer do anything that involved 3D processing and my LCD monitor started to flicker mildly.
Changed the cable for another standard cable and then for a DVI cable and the problem persisted. I even tried another monitor that I have with no change in this problem. That gave the card an effective life of just over 1 year.
I lived with it for a while, but soon I found that I could do very little, with full screen video only running for about 30mins before causing the system to hang. Even online maps and youtube started to cause me problems so I bought an x1650.
This was bad from the start as the POST screen came up looking all psychadelic and Windows loaded then promptly hung. Tried installing everything in safe mode (with no small amount of difficulty) and then rebooting normally I got a few minutes of operation before the system hung again. Returned the card to the shop and they confirmed it was faulty.
So I got another x1650 from a different retailer and got the same psychadelic POST screen, but at least Windows worked and allowed me to load all the drivers. 3D and video all seemed to work, then after a couple of hours the system started to hang again repeatedly.

I have checked all the card specs and checked everything in my motherboard manual, checked diagnostics using the BIOS for any voltage problems, temperature issues or conflicts, and there should be no problem (except that my 3.3v is only at 2.8v, but still within tolerances, and the AGP card uses the 1.5v input anyway).
After having 3 cards from ATI cause me problems, is it that they are just useless (despite being a market leader) or is there something else I'm missing?

I have an ASUS P4P800 board with Pentuim 4 3GHz chip, 2GB RAM, 400W psu (upgraded from a 350W 1 year ago cos the old one packed in), 1 HDD and one DVD-R. This system is now 4 years old being used solely as a home computer.