Take some caution though when trying to power 5+ cards. That load can apparently start melting the 24pin connector to the motherboard. I had it happen to me before, and I think it had something to do with maximum voltage pin 2 could handle or something like that.
I think it's a good investment so long as you buy low and sell high. The fact that people are exchanging real money to buy this "virtual" money is a good indication that bitcoin will always hold some kind of value.