Well first of all, 270 is not a lowly card as you put it, its on par with geforce 660 gtx and certanly is enough for both dolphin and pc games at 1080p.
Second, you 2500k is more then enough for dolphin too, just go for 270 and you will be fine
Second, you 2500k is more then enough for dolphin too, just go for 270 and you will be fine