I recently had a stick of memory go bad. I have a DDR2 system. It is about three years old and the memory I had was Corsair XMS2. The timings were 5-5-5-12. I had 4x1gb. I still have two that are fine but the problem I'm running into is that they discontinued the specific ram I bought, and all of the ram I am finding are in different timings. Do I need to keep timings all the same or would it hurt to have two 1gb slots running at 5-5-5-12 and then have two others with a different timing?
The MOBO will only run all of the RAM modules at the same timings (there is no way to set the separate modules to different timings) - so you will want to get a new set that is 5-5-5-12 or better ratings and uses the same voltage - A better rated module will be able to run at the looser timings but a poorer rated module may\may not be able to run at the tighter timing.
You want to keep the timings, voltage and Mhz all the same. I would not recommend mixing timings for your memory... you are just asking for Random BSOD. I would recommend 1) getting new DDR2 memory at 2X2GB or 2) run with 3GB of RAM. Unless you are running a 64-bit operating system, you will not be using all 4GB of RAM any ways, so you won't be hurt by going with 3GB's.
A little more info. I am running Vista 64. It seems that my ram was a bit of an oddball maybe. I did find another 5-5-5-12 stick but it was a different voltage. I think to be honest. All I currently want is 4GB of ram. Think I will just ditch the current stuff I have and get a 2x2gb like you guys said. I've had problems with my XMS2's. Had to RMA at least one when I was building this thing. Then no telling how long this one has been bad. I know it has been bad for a while because when I took out 2gb and noticed a speed increase =p. I'll just get 2x2 I think and later I can get another 2x2 if I feel the need.