Well, it depends. DDR is pretty much 100% phased out. Any new release mother uses ddr2 ram. It is a little better at overclocking. As time has brought it along, yes it is better. (cheaper too)
Even though it runs at 800mhz or 667mhz versus 400. The timings on ddr2 are a lot looser, and they can't handle as many volts. So it isn't an insane performance boost like you may have thought.
But from the looks of it you don't like to upgrading all the time.
, so I think you could go with a AM2 board, a x2 cpu and some ddr2 667mhz ram. You can get 2 gigs for under $60