I have a very similar system up and running now. I'm using an ABS Tagan ITZ series 800 W PSU (it is no longer available) and everything ran right from the start and some 2 months later after the build it's still going strong for the most part. I didn't go with the P5E3 Deluxe but the P5E works quite well with a QX6850, 4GB of DDR2 800 RAM, 2 HIS HD 3870's running in Crossfire (at least they both worked at first..more on this later)150GB Raptor HD, 2 x 500GB SATA 3.0 WD Hard drives, PCI Express X-Fi sound a pair of dual Layer DVD burners with Lightscribe all in a Cooler Master Cosmos 1000 case with Vista Ultimate 64 bit version.
Back to the video cards, Crossfire worked right out of the box with the pair of HIS HD 3870 cards but the one that was running as the slave died after about 3 weeks of light use. I RMA'd it and when the replacement card came I dropped it back into my system and nothing happened, it seemed to be DOA so I tried it in a working PC to make sure it was dead and not the pcI-e slot on my board, it didn't work so it's going back for a refund and I'll replace with one from another manufacturer. From what I was able to learn it doesn't matter if the cards were from different companies just as long as they use the same GPU. ATI/AMD just came out with version 8.1 catalyst drivers but for some strange reason they don't want to install properly so I'm waiting on a reply from tech support for a solution.
I did manage to play COD 4 but had to back the video setting down to what the game defaulted to for my system. I tried running it maxed out but after about 10 minutes of play the game would crash out to the desktop even with the most recent patches for it. The catalyst 8.1 drivers were supposed to help that issue but for now I can't say they will since I can't get them to install. Even at the default settings the game looked amazing and ran well. The only problem I ran into was if I played it for 3 plus hours my system became unstable and the 7.12 drivers went haywire on me but they did recover on their own. Everything is running at stock speeds so I figure it's a driver issue and ATI knows about it now.