On topic, you're not going to get too much of a boost moving from the 275 to 650.
Not a one to one with the 285 vs the 275, but those two cards aren't that far apart. If the original card is going bad, it wouldn't be a bad replacement since it runs cooler and takes less power to run.
Could you tell us more about your requirements, Mythy? What resolution will you be playing at? What games or genres? Still the system specs in your sig?