I did these very same upgrades myself. RAM is very easy.
The internal HD is harder. I purchased a nice 7200 rpm drive from Dell Home Business during one of their sales (~$130), added it to an external FW drive case, used CCC to clone the contents, set the external drive as boot disk to make sure everything is good, and then took it in for replacement via an Apple Certified Technician (identified as a recommendation from the Apple Store).
Made the system noticably snappier.