I'm going to be building a very similar system myself, so I feel free to pass on my thoughts about your proposed setup.
1) I don't recognize that power supply. Is there something stopping you from considering an Antec TruePower 500+ watt unit?
2) In my opinion, no, there isn't a reason to pay $500 more for the 4000 chip. However, the 3700 chip is not a lot more money ($100 or so) and that makes more sense to me. Or do what I'm going to do: buy the 3500 socket 939 chip and upgrade to a dual-core chip when they're released. They'll be going into socket 939, as far as I know (at the moment).
3) I'm going for the GA K8NSXP-SLI-939 board. More ports and more bells and whistles. Yes, it means you don't buy an outboard LAN card. On the board I want, there are TWO LAN ports. Onboard LAN, in this case, actually gives better performance than a LAN card, since the LAN port is on the system bus structure of the mobo. The board in my current system is a GA-K8NS Pro socket 754 and the onboard sound is a lot better than I thought it would be. I had bought the same sound card ou're thinking of, but ended up not using it. I'll put it into the nest system, instead
4) Don't waste the chance to put two 200 gig drives in your system. Or even two 250's. The sweet spot is around $100-110 per drive. 200's are in that spot now. Also, I'd get Seagate drives (SATA II) for the 5 year warranty.
5) If you get the GigaByte mobo, you have no need of outboard RAID controllers. The controllers and connectors are already onboard- both SATA and IDE.
6) If you feel you need Wi-fi in the system, then make it external by buying a LinkSys USB/Wi-fi adapter. It allows much more flexible antenna placement, which is always a good thing.
Personally, I'm going for four 250 gig drives for my upcoming system. I just like lots of room for growth (and games). I've had my GigaByte-powered system in place for a couple of months, now and have zero problems. I really love it. Then throw in the dual BIOS and online updating and it's hard to imagine a better setup.
<font color=green>****</font color=green> Never Assume <font color=red>ANYTHING</font color=red> <font color=green>****</font color=green>