for 450w, you could simply put 1 GPU for mining for the safest
Allocate - 330w for GPU's, 120w for the system
unless you will do underclocking and bios mod, and achieved 150w on each GPU you could get two
best GPU?
since you are into gaming with mining, better get the best for your gaming needs with those 8GB of mem, and that when mining is not profitable in the future you still have enjoying your rig