The thing is, minus 40 is a life-and-death sort of temperature, so you really need to know your stove is gonna light. So obviously, we're talking liquid fuel—white gas. Then, your stove's gotta be one that's really built to take extreme conditions, meaning an expedition-type stove. In this category, the most proven model is MSR's venerable XGK ($110, www.msrcorp.com), which has been around now for the better part of two decades. Multi-fuel capability, very rugged, extremely hot. Also noisy as hell with basically two burn settings—"off" and "lift-off"—but that's the price you pay.
The other good choice is the Optimus Nova ($149, www.optimususa.com). Optimus, of course, made the stoves that Scott and Amundsen took to the South Pole, so they know a thing or two about stoves and cold. The Nova is a fairly new design, but one built with extremely well-understood principles. Also multi-fuel, very well made.
I'd choose one of those two, period. There are cheaper stoves out there—and they're good stoves—but for the kind of camping you do, these are the ones.
Check out Outside's 2004 Buyer's Guide for the very best field-tested stoves.