If you don't NEED Crossfire, I'd say P45. Hoever, you'll loose the full 16x + 16x Crossfire and gain MHz on the FSB. If you do need Crossfire, x38/x48 is the way to go. Some say there's absolutely no difference between the two. I like to think that its the main reason why there's a huge price difference between the x38/x48s and the P35s/P45s. It'll give them something to work on later before the release of Nehalems tech.
If you're looking for a high performance board that can overclock your FSB for the future boards Asus is working, go with P45. It's the next best thing until the new Nehalem's technology comes out, should be anytime now.
I'm saving my money and to buy this new Nehalem's technology. It'll put FSB to shame.
But if you can't wait, take a look at the the Asus P5Q3 Deluxe WiFi, it's one of the best and affordable. If you want something that's cheap and overclock like crazy and have full xfire capabilities, go buy yourself a ASUS MAXIMUS FORMULA. Its probably the 2nd or 3rd best x38 board out there. It competes with most x48 boards, but it uses DDR2 mem. For an extra $50 though, you could get the ASUS P5E3 Deluxe/WiFi which is IMO the 2nd best x38 board. I think you can use either DDR2 or DDR3. PSE3 is one of my favorite boards due to its price and OCing ability.
The best x38 is the ASUS MAXIMUS EXTREME. However the Maximus Extreme II is in the works and that will support the P45 chip. It's said the FSB for Maximus Extreme II will hit FSBs of around 660MHz.
