Help Troubleshooting Graphic issues..

Ok. So here is what happens.

Everything is fine for a bit, then when I've been playing for an hour or so, sometimes it takes 4+ hours, sometimes it doesn't happen at all. Anyway, my screen freezes, like the pc is locked up, and after about 2 seconds my graphics turn all weird. Strange shapes, colors, video in the wrong place (Had my normal 'screen' show my movements and such in the 'U' map once) etc. and after such a glitch its obvious my computer is lagging a bit. But, with a minimize and restore video goes back to normal and I can continue playing.

Now, when I get done and logout, most times when I 'X' out of GW it causes my computer screen to freeze like its locked up again for a few seconds, then the screen goes black and unresponsive. From there I have to kill the power and start the machine up again. Sometimes this blackness won't occur and instead my desktop will look strange like GW did, with random shapes and colors, with a nv4_disp error, please restart for proper functioning sort of window.

I did some searching and ended up uninstalling my nvidia drivers all together, using a driver cleaner program and then reinstalling the drivers. This appeared to work fine for a few days of game play, until lastnight, it all began happening again. I am at a loss of where to go from here unless I begin trying older drivers. I have 2 computers with identical hardware except hard drive and obviously the windows install. Same MB, CPU, RAM (one is 2gigs, this one just 1gig, same brand and type however) and even the graphics cards are the same.

Hardware:
Intel 945XBX Motherboard
Intel 935 CPU
XFX 8600GS Video Card (I think, I'm 99.9% sure, I'll check tonight when I get home from work)
Corsair Dual channel DDR 667 Ram

Any ideas? Avenues to try?

You're seeing graphics memory corruption. My guess would be overheating gfx memory, but it could be a memory leak (buggy driver or helper program). What drivers are you using? If you're using drivers from XFX then instead try the ones from www.nvidia.com - and uninstall any helper apps from XFX.

EDIT: and if you're overclocking you gfx card, drop a bit lower.

No, no overclocking. Using latest nvidia drivers. Overheating? Possibly, I'll see if I can't track down something that'll read the card's temps.

Guild Wars is very heat sensitive, more so than most games of it's era. There should be a setting in the Nvidia Display Panel (or whatever it's called, sorry I'm more of an ATI guy) that will allow you to monitor the GPU's temperature.

I installed ntune, which allows me to monitor temps, and at idle its between 45-50c. I haven't had a chance to log some playtime yet to see how it handles load, but with 3dmark and ntune's stability tests, it got as warm as 64c. I used ntune to adjust the fan speed to 100% but I don't believe it stays there. I found a few other suggestions via google about overheating issues.

Also, is it normal for the cpu multiplier to bounce between x12 and x15 / which equates to 2.4 and 3.0(which my cpu is btw)? When under load it stays at 3.0, though when idle it constantly bounces.