Firstly, a couple of suggestions:
A 3800+ 64X2 is currently $313 on Newegg. Honestly, for $31 dollars more than the 3800+ 64, I think that is greatly worth it if you can spring it.
Check out the Thermaltake Circle case. Actually, it's "Thermalrock" but it's still Thermaltake...Newegg lists it under Thermaltake and Thermaltake has it on their website. It is about $140, with great airflow, has some slots for holding CDs on the front panel, and is absolutely massive. It is almost 11" wide, and is just under two feet long and two feet tall. Thermaltake's Tai-chi and Kandalf cases are only like 8.3" wide and not so long or tall, and much more expensive. Of course, if the biggest derned commercial case in the world is not to your taste, the Xaser V is a good pick (the aluminum one is best but there is a ceramic one for like $75...the plastic front panel is a little cheap though). The Xaser V is also very long. Length in a case facilitates airflow and ease of building more than the other two dimensions.
Thermaltake makes a good PSU, and they do have modular PSUs but they are expensive. Aspire makes a 500W psu with >70% sustained power, and dual 12V rail, with 16A on 12v1 and 18A on 12v2. The molex connectors are rather cheap, though. It costs 39.99 (I got a $10 rebate on it but that has expired now), and runs quite well.
I can't speak from personal experience with DFI. Naturally I have heard many good things about them, but I am a strong Asus/Abit fan for mobos. Alternatives to that DFI mobo:
Abit AT8
http://www.newegg.com/Product/Produc...82E16813127230
Abit Fatal1ty AN8-SLI
http://www.newegg.com/Product/Produc...82E16813127206
Asus A8R-MVP
http://www.newegg.com/Product/Produc...82E16813131584
(I dunno what the deal is with the reviews here. I own this board and have had no problems with the voltage going over 1.45, my raid0 array, or my dual X1800XTs).
Asus A8N32-SLI
http://www.newegg.com/Product/Produc...82E16813131568
Asus A8N-SLI Premium/Deluxe/Vanilla
http://www.newegg.com/Product/Produc...82E16813131540
Of course, these boards all have SLI support if you plan on going with SLI later.
But frankly, I would go with one of the boards with ATI Crossfire if you are going to buy a board with dual video card support. Why? Your 7800GTX costs $749. The ATI X1900XTX runs something more like $600, and the X1900XTX has been shown to run faster than DUAL 7800GTX 256MB cards (obviously running two of your BFG 7800GTX 512MBs is going to be closer because of the factory overclock and that you would have 1GB of video memory, but the fact that one X1900XTX is so up there in performance speaks volumes). I wish I had waited and not bought these dern X1800XT 512MBs...they're great, but I would not have minded spending $200 more for a dual X1900XTX system that would be even faster. I hadn't even caught wind of the X1900XTX when I purchased this rig...dern ATI!