When building a computer specifically for mining BitCoins, with ATI/AMD cards, is it neccessary to match the cards up like you would in a gaming machine, or can I simply plug in whatever card I have available at the time?

Right now I have a 6990 on the way, and I already have a 5970.  Can I just plug in those two cards without any special setup?  Additionally, could I add a 5870 to the mix as long as the motherboard / PSU can support all 3?