Mix different size memory modules in different slots?

Memory runs in banks. 2 slots make a bank, your board should have colored slots representing the matched banks. (Most of the time, same color slots are a bank, some boards you did use 2 different colors to make a "bank", check your manual to find out how your specific board pairs memory into banks.)

You want to keep the same size modules paired together in banks, if you do not, they will not run in dual channel mode. 2x1 gig in a bank, and 2x2 gig in another bank will run in dual channel mode. If you put different size memory together in the same bank, like a 2 gig stick, and a 1 gig stick into the same bank, they will not run in dual channel mode. If you put a single odd stick into 1 slot of a bank, your memory will not run in dual channel mode.

When you buy new memory, make sure it runs the EXACT same voltage and the EXACT same timings, and preferably the same brand as you already have. If you do not do this, prepare yourself for problems getting them all to work together.

I ran 2x512MB DDR2-533 (In dual channel) along with a single 1GB DDR2-533 stick. It ran perfectly fine, except it was then all single channel, instead of dual channel.