Wow..I had a very similar experience...the new Rosewill ram I bought wouldn't work in 2 systems with the same mb, a Biostar M7NCD, even though they're both rated to take pc3200 ram. These systems have been rock solid for the past year, but when I put the old ram back in, I started seeing having all kinds of problems, including the lockup one. I got everything else back to normal, but now, the lockup probelm seems unsolvable.
It's the *same* stick in the *same* slot on the *same* motherboard using the *same* power supply that worked flawlessly for over 12 months, but now I got a lockup whenever I throw 3d at the system. I ram Prime 95 overnight, and then did memtest for hours...no errors. As soon as I fire up UT, though, I'll get a lockup at some point, usually in the first 10 minutes or so. I've also tried Rise of Nations, and that locks up too, and it's not even 3d accelerated afaik.
I tried 3 different video cards: a GeForce 5700FX, a Radeon 8500, and a GeForce 3 Ti 500...same story.
I chalked it up to the motherboard...somewhere in the troubleshooting process, I flashed the bios to the latest one, and maybe it's causing the problems. I ordered a cheapo VIA kt266a board for $24 to test my theory.
I really have no clue...is just one of those weird computer problems you run into that makes you want to set fire to/shoot/blow up your stuff lol.