if they set it up so its 2 1 gig sticks and 2 512 sticks of RAM yes. that way it is utilizing the DDR (or possibly DDR2) technology. However, if its just three 1gig sticks, it will be a little slower do to the fact the memory will not be running in dual channel.
to kind-of answer your question on the mac forums about speed if not utilized as dual channel: i think (not 100% sure) that it will be slower. Simply because determinaning single/dual channel is done by the motherboard. Unless Mac uses wired mobo's i think it will treat it just like a windows system would. (if any mac user knows more bout this feel free to add)