RAM problem

[res0ndf7]

I just bought 256mb of RAM for my computer in addition to 256mb that was aready there. So i brought my current RAM(it was ddr400) in to Compusa and they said DDR400 was the same as 3200 so i got a stick of 3200 ram and when i put it in to my computer everything ran fine. I opened a game, no problems, then the game crashed and i tried again and the same thing happened after playing for around 5 minutes. I tried internet explorer and after awhile it had an error and crashed too. When i take out the new ram it works fine but when i take out the old ram and leave in the new ram it gets to the windows logo every time and then restarts......the rams are suppose to be the same and i dont think ones damaged.......there is a slight variation on the sticker on the side of each "DDR 400 Dimm,P 32WPD" and the other one is "DDR400 Dimm,Q 32WQD the guy at the store said they would work but they dont, can anyone shed a little light????