The DDR2 runs at much tighter timings. The reason I bring it up, is that you were looking at cost on the MoBo's, so I assumed you didnt have unlimited cash to toss at the system.
Going with DDR3 wont really make the Computer last any longer, as far as gaming goes. It will perform pretty much the same on a system with the same CPU and GPU.
People say, "you can upgrade later" but again, who knows if the older board will support the faster DDR3 and be stable, and your buying ram...again.
If you are trying to get the most for your money, get more HD space, or a faster GPU, or even a 2nd GPU.
Bottem Line, you wont see a real boost when gaming, or anything for that matter, you more than likly wont upgrade your ram, and both computers would pretty much be replaced at the same time. It wouldnt even get you ~3fps, if anything.