Basically can computers support 2 different types of ram? Say I had PC2100 in teh machine and wanted to add a stick of PC-2700. Is it all dependent on if the MB supports it? WinXP supports it? Will the 2 different speeds work together? Thanks guys.

Most motherboards should run the 2700 at 2100 automatically.

Ahh, so the MB will run the ram at what it was designed to run it at. I see thanks...=)

Like Sanchek said, what happens is that your mother board downclocks your 2700 to the 2100, so in essence it'll be like running dual 2100s

Ahh, so the MB will run the ram at what it was designed to run it at. I see thanks...=)
Not exactly.
You motherboard will have a MAX bus frequency to run the RAM at. If you exceed that, then yes it'll downclock the ram to the "frequency it was designed to run it at".
But unless that max frequency is at 2100, then that scenario wont apply to you in this particular instance. Instead, what happens is that the motherboard downclocks your fastest RAM stick to the slowest stick's frequency, so that they'll all run at the same speed.

You can have 3 sticks with different speeds in the same motherboard, and what'll happen is that they'll all run at the slowest stick's frequency.

Kiv is correct.

Simple translation:

Yes, you can... but unless you already have to 2700, theres no point in buying faster ram if you're going to keep the slow ram. If you already have the stick of 2700, that should be fine.