As far as I know, you cannot do this. First of all, your motherboard has to be able to support PC3200 RAM. Check your motherboard specs. Second of all, as far as I know, if you mix PC2100 with PC3200, all of the ram will run at the speed of the slowest memory - which means it will all run at PC2100.

stay with the same speed
not to say (RTM) But Read the manual and see what your motherboard supports

In all reality, you can buy whatever speed you want, as long as it is at least PC2100. The higher speed stuff will downclock itself to match the lower speed.

And if you have to ask about overclocking, you're one step closer to finding out how to get more bang for your buck. Chances are, you own an OEM machine that has very propietary hardware, which means the chances of you overclocking are pretty much nil.

Anyway, buy whatever you want, as long as it's at least PC2100.

I recommend Crucial brand ram.

www.crucial.com

Oooh, I knew I was right about the memory slowing itself down.