Well, as far as I know there is a couple of DDR-ram P4 mobo's around. (Not sure if it's only the VIA chipset that's available). As it seems, DDR-ram don't get much lower benchmarks than the RDRAM setups, making the DDR-ram setups much more price-worthy.
As for the temparature issue, I recommend you getting an Intel processor. AMD processors are known to be bad at handling excessive temperatures. But still, the Intel processor will not perform the way it's supposed to if it's too hot. So I suggest that you get extra fans for the casing, or even getting some sort of water based cooling for your system. I've heard about reasonably cheap water based cooling systems that would make your system more stable in the harsh enviroment you are running at. And, if you're thinking of overclocking the processor, I strongly recommend such type of cooling.
I'm running a server computer (for web-services and such) at a server hotel. They have many servers running there, and since it's quite important that the servers is up 24/7, they need very good temperature monitoring. Well, this might not be appropriate for a home setup, but they have a big air conditioner stationed in the room, which is set to have the room at around 20 degrees celsius at all times (I think that's the setting they're running). And they have 2-3 fans on each casing for every server computer they're running.
I hope what I told you was of any use.