The basic graphics card formula is to find, plotted on a graph, the best performance to price ratio. You might have other desires too, like size, noise, or features. I read reviews to decide which card to get. My card ended up not supporting a very old CGA/EGA mode I wanted to use - and assumed would be present. No review is going to mention a thing like that, though. A good starting point for reviews is tomshardware.com.
I don't know what sort of power your existing choices pull, but IMO no more than 500W is necessary. If you really go bananas with the graphics cards or peripherals, you might make use of more, but this is uncommon.
Personally I would go for a solid state (fanless) 500W power supply. Lovely.