If you get a chance, check out the current (Jan 04) issue of CPU Mag. Not to plug my own workplace or anything... =P
There's a pretty good write up on the BTX form factor. It looks like it'll be a lot more efficient @ cooling and such. The CPU, RAM, Northbridge will all be located @ the bottom area of the board. The ide/sata busses, PCIX/AGP, and PCI will all be located @ the top. ATX cases will NOT be compatible w/ BTX mobo's. Although, an ATX PSU will work on a BTX mobo.
From the schematics I've seen, the cooling solutions for BTX look much more effective. It almost looks like the CPU's hsf will have a fan that's mounted on the side of the hsf, as opposed to the top, drawing air in from the front intake fan of your case. The fact that the CPU and all the other components that make the most heat are at the bottom of the case looks good too. These comonents will get the bulk of the fresh cool outside air coming in from the bottom of the case...
That's all the info I can think of atm. Check out the CPU write up though. It's a worthwhile read if you're really interested...