So, how important is transfer rate and seek-time? You've been researching stuff at a furious rate.
Hmm, DDR3 is still rather pricey. OTOH, DDR2 is dirt cheap now, which was not the case just one year ago. That's up to you, do you want to be leading-edge or trailing-edge?
Leading edge is expensive, trailing edge is for cheapskates like me.
When I built this computer less than one year ago, I got a board with slots for both DDR and DDR2, as I wanted to use some sticks of DDR that were (are) still good. At that time DDR3 was not yet available. Who knew?
When people talk about building something that's "future-proof" , I admit I'm skeptical.