Okay, I have 3 DIMMs, 2x256 and 1 512, obviously running in single channel. If I get another 512 and it has the same CAS latency but it's NOT by the same manufacturer as the other 512, will both channels still run in dual channel mode? Thanks.

You'll have to check with the manufacturer of your Motherboard. Some will, some won't, usually the older ones won't, but I may be incorrect. You will have to be sure you install the DIMM's correctly so as to pair like modules.

Generally you dont want to even combine 2 different amont sticks on the same board. I know with crucial they like to burn that way.

most boards tend to get a little bit pissed when you fill all 4 dimm slots.

what chipset are we looking at? Are you saying that you are taking out the two 256MB sticks and replacing with one 512MB?
I would say it should work just fine. Here's what one of my buddies said about his memory setup today. He runs a nforce2 setup.