Just to clear up any confusion, the Pentium D and Core 2 Duo are both dual-core processors. Pentium D is not the same as Pentium 4. And yes, the C2D 6300 (1.86 Ghz) blows the Pentium D out of the water.

400w PSU should be fine depending on what kind of video card(s) you plan to use. If you want to go SLi, think higher. But personally, I think it's too much trouble (and expensive). Just get yourself something like a 7600 GT for a nice cheap price and save up for a Direct X 10 card when Vista comes out and all the prices drop. Don't get that Gforce 8800 unless you want to start hemmorhaging money right now.

I can't speak to RAM modules, but I haven't had any problems with budget brands. Still, if you have a choice, go with the value series from reputable companies like Crucial, Corsair, etc.

As for the optical drive, I recommend finding a nice, cheap one like the NEC 3550. That thing costs $30 and I have never heard any complaints. It burns fast and reliably, and is compatible with just about anything.
I personally bought a Samsung 18x drive for the same price ($30) that includes 12x DVD-RAM and Lightscribe. It's as fast as any other drive on the market, and has more options than I know what to do with. Plus, it comes with Nero burning software