iq1:
Wait for the next quad core from Intel based on 45nm.
Wait 3-5 weeks for the specs/benches on the ATI R600.
Get the ARECA raid card. Use 4+ RAPTORS in a RAID0, and store any crucial data on a network drive.
Get 2-4GB of Corsair Ram. The only reason to not use Corsair is price; not important to you. Also look into thier newest dominator line-up; I think I saw some C5 stuff???
Definitely stick with a non-workstation motherboard. You want to be able to use dual video cards in 16x PCI-E slots for the best results.
Noise level output from your Powersupply should be a concern. Make sure you get something both powerful enough and quiet.
Get a watercooling kit from DANGERDEN. Use this with your 8800GTX BFG Pre-Waterblocked video cards (or ATI R600s).
Get the best case you can find. Big. Shiny. Silverstone, Antec, Coolermaster, Lian-Li, etc. All great names. I really like the Coolermaster Stacker CM830 series stuff. The Silverstone TJ-09 is really nice too.
Overclock the quad-core proc until it becomes slightly unstable, then reel it back in a little for stability.
Definitely get the best sound card money can buy. XFI plat or whatever.
Get the Logitech Z5500 speakers. Or go crazy and get a Onkyo Receiver and spend another $10k on decent speakers/subwoofer.
Whatever you do, don't use an HDTV as your monitor. Get the 1080p Dell 27" monitors (2 of them). Or wait for the new 30"s with the good response times.
Look into getting a UPS - these filter the power being fed to your system and can extend it's lifespan. Worthwhile with so many high end parts plugged into it.
I've tried almost all the gaming mice, and I still prefer the Logitech G5, although it's last years baby.
Get the Logitech G15 gaming keyboard. Its sweet.
Don't get the Xeon/Opteron rigs. They sound great on paper, but they are really meant for workstation-level jobs, like serious number crunching for scientific research or crazy-busy fileservers with 100s of users requesting I/Os. These rigs have expensive motherboards, have limited upgrade paths, require ECC ram, don't have a full feature set, don't have SLI, etc.
Finally - tell us what you decide to do!