I have 2GB (2x 1GB) of mushkin DDR2 ram in two slots on my mobo and I'm thinking about throwing in a 2nd pair of DDR2 into the remaining two slots. The problem is the mushkin is 5-5-5-15 timing and the other two modules of ram I have are 5-5-5-12 timing. Is it possible to install all 4 modules together? Thanks.
It really depends on your board. Some are very, very picky about running 4 sticks. Some are not. Usually though, if they work on boot, your board sets the memory timings to the speed of the slowest set. Some boards will automatically lower the speed of your memory if all 4 slots are populated just because tight timings are hard to run in all 4 slots.
But, everything aside, looking at the timing of your memory, I'd say that everything should work just fine.