You most definitely can, in fact I do that when gaming...leave my 6950 chugging along in the back while my 7950 runs the games.
Just remember to asign one core to the mining program (core affinity in task manager)
If you have cheap power bills, yeah, for sure...if you have one already. If not get 7950s, or cheap used 6950s and flash to 6970. Those are the best value atm.
I'd wait to see what the 8970 and 8950 have to offer though.
FTC has been like that for a while now, like two months...I'd pass on it and just focus on other alts with a brighter future. (digital coin, litecoin, world coin, mincoin maybe?, franko, hyper coin)