the p45 board CFs at 8x8 or runs the full 16 for a single card, but keep in mind that they are PCI-E 2.0 slots, giving them substantially more throughput than a standard 8x8 connection. The reviews I've seen bear very little difference between a true 16x16 and the p45's 8x8. it seems to depend on how much memory the vid card has. check out reviews for yourself and see
$300 Rampage + $300 HD 4870 1GB is a waste IMO. $120 P5Q Pro + $550 HD 4870 X2 is a lot better for almost the same price. If you can't afford $670 now then at least get the P5E Deluxe, it's still X48 and cheaper. This way you end up paying $220+$300+(hopefully) only $200 for the second card later, so you waste only $50 and get the real performance later but you don't need to have all the money up front.
If you're not dead set on overclocking, then you should go for the P45. Much cheaper than the Rampage and has all you need. The biggest difference that sets both apart would be the Rampage's overclocking features, and you wont want to pay a lot more for such features which you might not be using at all. The X48 provides full X16/X16 on Crossfire but it's not significantly better than X8/X8. You know what you want, don't pay for what you don't need.