A couple days ago, I bought a some more ram for my computer. I tried to buy ram that was exactly like the stuff I already had, but the new pair I got had slightly different timings than my original pair (I'm not sure if this really matters though).
So I installed the ram, and my computer wouldn't start up (this was on xp). I didn't really think about it at the time, so I went ahead, took out the newly installed ram and then went and installed windows vista (which I had also just bought).
Once I got that all taken care of, I tried to put the new sticks of ram back in and got the same result as before. If I have both pairs in together, nothing works, but if I use either separately in either pair of slots on my motherboard, everything is fine. I have used the windows memory checker thing about 6 or 7 times now, and each time it confirmed that everything was fine (except for the time when I had both pairs of ram in, it immediately found a hardware error that time).
The old ram is 2 x 1gb Corsair XMS with 5-5-5-12 timings.
The new ram is 2 x 1gb Corsair XMS with 5-5-5-18 timings.
I've checked my motherboard and it supports up to 8gb's of memory.