Judging by your choices, you seem to have a good handle on things. Big thumbs up on the HDD choice: 320GB/platter and only 2 platters is probably the best compromise in terms of speed, capacity, heat and noise.
I don't know how picky you are about noise, but if you are like me you should consider going with 1200 or 1600rpm S-Flexes instead of the Ultra Kazes according to this review at SPCR. The only situations where an Ultra is needed is if you're trying to push air through thick filters or something that needs a lot of pressure.
On the RAM issue, I personally went with 800Mhz modules since they are so much cheaper and I don't think the the performance difference is noticeable outside of benchmarks specific to mem bandwidth.
As for video, depending on how long you keep your components for, you may want to join me in waiting for the 1gb version of the 4870, although I feel guilty for suggesting the waiting game since there really is no knowing how much longer it will take and no guarantee that they won't be as ridiculously priced as the preorder ones ($350-400). But if you weren't planning on using Crossfire (since you suggested getting a 280), there are some amazing deals to be had on the GTX260- I've seen it as low as $240 after rebates last week.
It's probably just me, but I'd skimp on the RAM (speed & brand, not quantity) rather than CPU to free up $ for the vid card. It's not like the pc2-6400's don't have any OC headroom to get up to similar speeds as the ones you were thinking of getting if you really felt the need to run them at that speed. And definitely DS4 over the DQ6.