this has been discussed many times before, and for some its worth it, for some its not.
sd card is not very fast, so your computer will be very very slow. sd has limited write cycles so if you do not have EWF installed (which is also a complicated procedure) it'll die in a short time.
Your next bet is CF card which is faster, but still not as fast as normal HD, and it also have write limits which means u also need EWF.
The reality of it is, PRICE (CF + Adapter) = PRICE (SSD) nowadays. However no one has yet done anything testing on SSD in cold weather. I myself is one pioneer among others, testing ssd in car to solve cold issue. Last year i tried using patriot ssd, which failed majorly. This year i'm using OCZ ssd, which haven't been fully tested yet because we haven't hit below -10*C yet in calgary this year. More information will be available this winter as more ssd have been tested. Check the environmental section there's a thread dedicated for this info.