You may have to overclock your CPU a bit to get the most out of something in the R9 290 performance range, though it would depend on the games you play, less CPU intensive titles wouldn't be much of a problem. A 500 Watt PSU may not be enough unless it is a very good quality unit, the Hawaii GPUs are very power hungry and I personally wouldn't put them into any system without at least a good quality 550 Watt power supply or better, especially if you're also going to throw a CPU overclock into the mix.
You may be better off just saving for the GTX 970 or waiting to see if AMD's new cards are a little more power efficient, as going with the 290 may also have the added cost of a new power supply as well.