Run a Memory Test diag, like the MemTest86 I link previously. If it crashes while testing, try slowing down the RAM speed. You'll have to check your BIOS settings for that. And, if you're not sure how to do it, check your motherboard manufacturer's site, and RAM manufacturer's site about specs, and/or google setting BIOS RAM settings.
If it doesn't find any problems, and doesn't crash, then the RAM is probably ok, and it was just a fluke crash.
Once you've ruled out the RAM, make sure the old video card will works as it's suppose to. If not, there's probably some driver problems, that need to be addressed due to the switching of the cards. Once it appears the old video card is working fine, replace it with the new one again, and set it to it's minimalist settings in the driver, and in EQ, and see if it helps.
If it doesn't do anything different (i.e. still crashes after 10min, probably a problem on the video card. If you gain some time, but still crashes, try removing the side panel on the case, and see if that gets you even more time out of it. If so, it's a heat issue, and several ways to get around it. Get a better heat sink / fan for the video card .. get a slot fan .. get another case fan .. etc., or just send it back and get a cooler running video card.
EQ shouldn't really be that taxing on a video card of that caliber, and shouldn't be overheating, unless it was poorly designed or used under-rated cooling components (generally happens with 3rd party video cards to save money).
If you think it's worth keeping but still having trouble, try searching for alternate / older drivers, that have been reported by others to work well with EQ or similar games.
That's about as much as I can recommend. If you still can't narrow the problem down, I'd definitely try as Jonwin said and post on the EQ Tech forums, and maybe even the Nvidia (or the card's manufacturer's if not Nvidia) tech forums.
Hope you get it solved and working!
Yther Ore.