7800GTX is just the best to get right now. I just wish I had the extra $200 to get it. It's easy to get carried away when building a system, especially when you want it to be good. Dual channel is a basic config with that motherboard so you can get any 2x512MB RAM and it will automatically be in dual channel (if you put them in the right slots according to the motherboard manual).
I would downgrade the monitor, speakers, graphics card, motherboard, CPU, and the sound card. You can use the onboard sound temporarily until you have more money later to add the X-Fi in there. The X-530 is a nice set of speakers.
Logitech X-530 70 Watts 5.1 Speaker - Retail
DFI LANPARTY UT nF4 Ultra-D Socket 939 NVIDIA nForce4 Ultra ATX AMD Motherboard - Retail
AMD Athlon 64 X2 3800+ 1GHz FSB 2 x 512KB L2 Cache Socket 939 Dual Core Processor - Retail
SAMSUNG 940b-Black 19" 8ms LCD Monitor 300 cd/m2 700:1 0.294mm Pixel Pitch - Retail
For monitors, they're almost all the same price if you plan on getting LCD 19 inches. That DFI motherboard is almost $200. Maybe get something around $120-150. It still doesn an excellent job and you won't even notice a difference in performance. If you still plan on getting an SLi motherboard, get a 7800GT video card and buy another one later down the road. DO this if your budget is tight. The 7800GTX is a nice video card and highly recommended, but like you say, at a higher cost. Eventually, it will come down in price.
For memory, Corsair XMS and OCZ Gold or Platinum are two performance RAM I feel are good. I've been using Corsair for so long, that I may end up getting Corsair XMS again.
Even after all those changes, you will still have a kick ass computer and at the same time, shave off hundreds and hundreds of dollars doing it.