DDR is by far slower than ddr2, and its not more expensive at all.
Also, @ Trifid, Mixing memories even from SAME brands, just diffrent models, or even similar models and just diffrent diffrent revision can cause stability issues to major issues that might prevent from computer to post, It is recommended by all of the big brands to buy everything you're going to need in advance in order to prevent those upgrades since sometimes they will only do you problems.
However while using same brand and same models that were just bought on diffrent times and are probably diffrent s/n or revision, the stability issues possibility is lower than mixing two diffrent models of ram or brands.
-Jo.