Usually most boards are fine as long as the CAS is the same timing..
For instance in my computer i can run 1 x 512mb Geil PC3200 and 1 x 512mb Corsair PC3200, however if i try and run either of these CAS 2.5's with 512mb Kingston PC3200 CAS3 it really doesn't like it.
Nforce 2 / Nforce 3 boards tend not to like CAS3 memory timings for some reason, personally if your going to buy some more memory you might as well buy exactly the same or as near as possible, it's not going to cost you any extra.