You will either have the 375 watt or 425 watt power supply. You can open up your side panel and the information should be displayed on a sticker on the side of the power supply. Even if you have the 375 watt, you will have the additional 6-pin power PCIe power cable for either the HD 7770 or the HD 7790 and they will work. Your total system power used with either card is only going to be around 250 watts or less.
I am surprised about the HD 4650 since it was never considered a gaming card even back in 2009. The popular card was the HD 4670 for gaming. HERE is a popular HD7770 at a good price with a free game. If you're happy with the HD 4650, then you should be thrilled with the HD 7770 and probably don't need anything better.