The max you can run is a Phenom II X4 940 3ghz, your 805 is at 2.5ghz the difference isn't worth dropping $40 on. You can overclock what you have stable to 3ghz easy enough on the stock cooler and maybe higher with a aftermarket. For Minecraft just about anything will work but Fortnight you want a CPU that stays above 3ghz and the best GPU you can find. A 750 TI works pretty well for both games, has a low power draw and you can find fairly cheap. The power supply is going to limit the options as you don't know the quality and the Legacy BIOS makes newer GPU's troublesome because there are no guarantees they will even fire up.