Very contructive, especially considering you don't even know what the primary uses of the computer will be.
What are the differences on cost? I would recommend going the ddr2 route, but I tend to keep my PC's a LONG time and want to be able to future proof as much as possible. In the short term, I don't think there is a really big performance difference either way.
PS: As said below, if you go ddr, there is realy no reason going ddr500, I would just get ddr400. That will put it at 1:1 with fsb and presumably better timings.