1) You are not obligated to run SLI. The board will allow you to run a single GPU (nvidia or ATI) fine.
2) ATI GPUs will work fine, however only one. To run multiple ATI cards you need a Crossfire board.
You can save some money if you go with PC2-6400 RAM instead of the 8500 RAM you've selected. The PC2-6400 stuff is plenty fine, and cheaper.
The Antec power supply is fine, assuming you don't necessarily want to run SLI or Crossfire on the latest video cards. The latest offerings are more power hungry (some new nVidia cards for instance need 2 power connectors EACH). So for future use, you may want a better PSU. Antec is good though.
Hard Drive: Not sure where you're shopping. However, I noticed TigerDirect.com this weekend has a 1TB and 1.5TB Seagate hard drives for cheap. $99 for 1 TB, and $119 for 1.5TB if I recall correctly.
Edit: You posted prices. Goodie!
Your case is listed on NewEgg.com for $79.98 plus a $20 MIR. So it's $59.98 there. Plus free shipping on it.
Looks good to me, but imo unless you plan to SLI in the future get a P45 board. Also do you plan to OC? Consider dropping down to DDR2 800. I find that most DDR2 1066 RAM have compatibility issues. Get some DDR2 800 that runs at 1.8v. (ie Muskin)
Generally Intel chipsets are considered to run more stable, cool, etc., compared to nVidia ones; I would ONLY buy a nVidia motherboard if I already had a good nVidia card I was going to SLI for sure.
That said, in your position even though you would like to add a second card some time next year, I'd still recommend the Intel P45 motherboards (Gigabyte, personally). Use your 8800 for the time being and then switch to a 4850 in 2009 when they're down in price, possibly get two at the same time or spread them out. This will provide longer life to the system than using two 8800's, plus you'll get the stability of the P45 boards.
I also agree about dropping down to DDR2-800 RAM, the only difference between it and DDR2-1066 is that DDR2-1066 will overclock the processor further. DDR2-800 will take the E8400 to 3.6GHz, and quite possibly without even increasing voltages on the processor. I know you mentioned you aren't interested in overclocking for the most part, but really it's not overly difficult and there's plenty of guides out there that break it down nicely. Anyway just some food for though!
I think you're right, but unfortunately I ordered all the parts. I didn't want to lose the savings from black friday and what not, so I was in a rush. I didn't drop down to ddr2-800 nor did I change to a p45.
I feel the mistake won't be too harsh. I mean, if there is a drastic difference, it'll be my first bad computer buy. Yay. Foolishly spent money. Yay.