I've seen a system running on RDRAM @ 800mhz on a 850 based motherboard recently but the fsb was @ 533mhz (P4 2,4) and i was really impressed by the extreme speeds of the whole system. Is there any other manufacturer supporting further clock speeds (or HT)?
Seriously look at the dual DDR 400. RIMM is dead (for many reasons - excessive liscencing fees, finicky stability, etc.).
DDR has set the standard for memory and it's gonna be here for a while and prices will keep dropping as volume increases. If you get in bed with RDRAM you are taking price and avaialability risk.
As for chipsets. ASUS has publicly admitted it's mod'd its boards to make the 865 chipset run at 875 performance. (Rumor, and benchmark results, indicate Gigabyte is doing the same but is not admitting it.) Avoid MSI. My friends and I have all had manufacturing quality issues (improperly installed heatsinks, cold solder joints, parts not flush on the boards, etc) with MSI products (MB and video) in the last year.
6-channel s-ata raid... gigabit... and check out that built-in dedicated exhaust fan for the power regulators and capacitors! it's just awesome. wish i had lots of extra cash, i'd put together my first intel system with that board and a 9800pro. just for a different flavor.