Is your memory running in dual channel right now? if so, will it be able to run with 3gb in each channel? Otherwise your computer might be noticably slower because of this?
The large amount of cache on the intel's (especially the quad cores) would negate most of the performance drop from going to single channel.
OP: You still should be able to run the 1gb sticks and the 2 gig sticks in dual channel in anycase. However, its not worth it in the long run i dont think youd notice much difference at all.
You can check slot usage with CPU-z