I'll go ahead and ask the obvious before anyone else does.
Have you patched EVERYTHING, and by this I mean: the game; the video card; direct x (9c?); sound card drivers; anything else you can think of to update. Windows AutoUpdate would be good too, as it's possible that you've updated a driver or DirectX and it's trying to use something in Windows that isn't updated yet.
Does this happen to any other game or program? It's possible that you've got faulty RAM, I've been told that this stuff's increadibly tricky to diagnose at times since it can literaly look like any other number of things going bad. The only way you'd find out if it was RAM related (again, from what I'm told) is to swap out the memory, do whatever it was that caused problems, then not have it happen. That or find someone that can acutaly diagnose RAM sticks (ie: computer store)
I realy doubt that an X300 is under powered, I played it on my old P4 2g box w/9800 Radeon (128m) with little trouble, and I use the biggest maps too. Sure a few of the late-game battles on 3+ sided games got a bit kludgy, but that was everyone throwing everything into a monsterous multi-sided war. I would encourage those as they made REAL fun Nuke Targets. I'm way too bloody fond of the A-bomb for anyone's peace of mind.
Great, now I'm going to have to install this on the new box and see how it runs. Heh, lemme know, mabye we can try a MP game.