You say you "upgraded to 1.5GB" of RAM - does that mean you're using 3 sticks? If so, are you sure your motherboard actually supports 3? (I know mine doesn't)
Have you tried running Memtest (http://www.memtest86.com/) to see if there are any RAM problems? I was getting many crashes since getting this computer (2 years ago!), ran that and it found issues, and finally have replaced the RAM so the PC is far more stable now.
Unfortunately there doesn't seem to be a similar utility for testing VRAM.
Are you able to try that video card in another computer/another video card in this computer?
Well, I know my motherboard supports 3 sticks, but I'm actually using one 1 GB stick and a 512 MB stick. I didn't pair them, but that's what you get when the 1 GB stick is given to you as a present...
I'm not quite sure if it's the memory that's causing the problems though. I upgraded it way back in December two years ago? Maybe one, not quite sure. But I hadn't noticed any problems right after the upgrade. I guess it doesn't hurt to test it out and see if that's the case...
As for trying the GA in another computer...the only other computer in the household is too ancient to handle my GA, so that's kinda outta the question. All my friend's computers run PCI-E now too so...yeah.
Update: As per Mxy's suggestion, as soon as I booted up my computer this afternoon, I loaded up Halo. No freezing or anything. I did pulled up Halo again about 10 minutes later, it's fine too. I'm going to go do some things around the house and maybe work on some stuff in Photoshop then I'll try Halo a bit later on at night.