Ok, I recently built a new system w/ the Asus A8N-SLi Premium mobo and the Athlon x2 3800. When I first bought RAM I got: ORSAIR XMS 2GB (2 x 1GB) 184-Pin DDR SDRAM Unbuffered DDR 400 (PC 3200) Dual Channel Kit System Memory.
Well, I'm looking to add another 2GB. But was curious if was possible to use a different PC Speed (PC4000+) since I'm using PC3200 right now.
Yes but it's pointless and you may run into stability problems. The entire system bus will run at the slowest speed RAM... in other words everything will operate at DDR400/PC3200 speeds even though you have PC4000 DIMMs. Also, if the timings don't match you could run into stability problems.
My suggestion is to just get another matched pair of the RAM you already have. Otherwise you're wasting your $$$.
You can manually set the speeds of the memory, and Corsair XMS memory can run a good bit faster than PC3200.
So as long as you buy RAM with as good or better timings than what you have, you can probably run at whatever speed you can run now with, minus any penalty for 4 sticks you might encounter.