Also keep in mind that two 470s will probably be noisy. According to that 3dGuru review, it's 55DB @ 75 CM, which they refer to as "quite something". I suspect two 5870s or 5850s would be a bit better, though still worse than a single card. Still, if you're planning to go CF or SLI, that's something you've got to expect.
Personally, I'm really reticent to go with current nVidia cards... I don't really like the whole power hungry thing. Doesn't seem healthy
AFAIK, it goes like this: 2x 5870 > 5970 > 2x 5850. I'm not exactly sure where 2x 470 would fit in there, though.