It is my advice that you avoid DDR2 at any and all costs, until the transition to DDR3 or XDR RAM takes place. DDR2 has impressive speeds at the moment, but is notorious for horribly high latencies. If you don't believe me, just look at VRAM, GDDR2 was virtually skipped over because companies recognised the high latency issue and switched to DDR3, which corrects this problem.
XDR RAM is the best, boasting DDR3 speeds with DDR1 timings (supposedly), but with Rambus spearheading this technology it may be difficult to force upon the market, especially if it is pricey.