Dual Channel -> Can use DDR of different sizes?

I am using an Nforce2 MB. The MSI K7N2G-ILSR. Can I use a 256 MB DDR333Mhz DIMM on banks 1 and 2 and a 512MB DDR333Mhz on the 3rd bank? Will Dual Channel still function in such a configuration? I need to expand my RAM but would like to go all the way to 1GB without throwing away my existing RAM.

What are the rules for Dual channel? Other than having RAM of the same timing? Thanks.

I thought that you get the best performance if you set it up like this 256 and 256 in dual channel together and then add the 512 stick as single channel ext to it, i don't know how this would pan out in you RAM slots as i'm not familiar with that brand Mobo

so 256-DUAL-256 512 for a total of 1 gig