DDR and DDR2 are not interchangable. Even if you use a bigger hammer.
Your PSU may be the problem. Just because it says 400W doesn't mean that 400W is what it actually puts out. A Google search for it only pulled up the company's page for it, so that tells me it is a cheap one. I'm not up on PSus, but I am sure someone could tell you a decent one, or you can check the Recommended PC Components thread for an up to date list.
As for RAM, I assume you are running XP, so you can only utilize 2 gigs. if your motherboard is dual-channel capable, a matched set of 2 x 1 gig would be the best thing for RAM. While RAM is pretty cheap, DDR no longer is. Everyone has moved on to DDR2, so DDR is basically what is left.