There's strength in numbers 😀 I'm trying to avoid repeating my previous home-build experience where I went a bit "exotic" ...
The graphic card question is really tough. I'll I can say is WAIT! Both cards are spanking new, the GTX-275 is just coming into stock. The 4890 is attractively priced, but the prices will likely come down for the gtx-275. The features that distinguish the cards are ex 10.1 vs PhysX, but those are a draw (so few games take advantage of either of them). There may be fewer headaches with nVidia drivers, but you'll get better "bang for the buck" with ATi cards. I've heard good things about EVGA so that's tempting. Other than that, I'm also completely confused... For a while I was really tempted by the gtx 285, it seems to be the reference standard for the other cards, but the price is way high on that one.
For now I'm just moaning about EggHead. Royal mess up on their end with shipping out my order. It'll be almost 2 weeks if I wait, or I might cancel my order and buy everything at microcenter. They don't have the single-card mobo I list above, but they do have one of the ones you listed at a reasonable price:
GA-EP45-UD3P
...and that would commit me to the 4890 as you say, but I think crossfire is more stable on these setups (from what I've heard, you have to go I7 for a stable SLI system).
OTOH I noticed microcenter has a gtx-285 going for $295 after rebate. Almost low enough to think about 😛