Which asus mobo should i pick?

I'm confused, there are so many asus mobo's and I'm really not sure which to get, Asus mobo's have treated me well so far so I'm sticking with them. Here's the deal: I want to run a E8500 with a sapphire hd 4870 1gb. I will be running a 64bit os with 8gb of ram... does that matter? I would like to get a second 4870 down the road so i'm pretty sure i want to go with either the p35 or p45 chipsets, can someone explain the difference between

Thanks for any help you can give me.

P.S. I was going to ask in the other forum but if anyone knows here please chime in, i have a pc power and cooling silencer 500w power supply, will that be enough for that card... i know i'll need a bigger one when i crossfire, thanks.