I built my pc last week and I'm having troubles with little flickering squares / textures stretching off into the distance when I play games like warhammer online, fallout 3, dead space. However on Age of Conan there are no problems at all. I've been searching for the past couple days for solutions, most of which blamed overheating / outdated drivers. I've checked the heating issue: my gpu reaches 50c and my cpu gets to ~45c when playing fallout 3. I have updated everything I could think of: video card, directx, motherboard bios. The only solution I have found was to alt-tab, which got rid of the glitches in fallout 3 and temporarily fixes dead space and warhammer online (for about 10 seconds then its back).
Processor: AMD Anthlon II 530 Processor 2.80 GHz
Motherboard: ASRock M3A770DE: Socket AM3, AMD 770 Chipset, ATX
RAM: 4.00 GB
OS: Windows 7 64 bit
Graphics Card: Radeon HD 4850 512MB
Power Supply: Antec BP550 Plus 550W
@Wa1, that was my very first thought as soon as I saw the PSU. That Antec is a modular PSU (I have a huge pet peeve about modular PSU's, but I'll leave my emotions at the doorstep on this one).
It could definitely be a faulting PSU as the Power Supply you have should be fine with the requirements, although it is a little on the low side (A 650 watt would be a tad more reassuring). I would test it with a different PSU first, and if that doesn't fix it, use a different video card.
If switching the PSU fixes it, then you need a better PSU or your current one isn't working right.
If switching the video card fixes it, RMA your 4850, it's bad
If neither of those fix it (that is, you tried two different video cards so the chances of driver issues on TWO cards causing the same issues is slim to none) then I'd go so far as to say that your mobo is acting funky. As you stated in your post, it's not a heat issue, however as an FYI I had this happen to me with my GTX 285 in Left 4 Dead 2 (max settings 1680). I increased the fan speed to maximum and the problem stopped.