You've not specified your budget; I'm assuming $200; maybe a little more?
Among the highest bang/buck cards right now are the HD7850 and HD7870. They are not power hogs, run well at stock but also overclock well, and do not generate a lot of heat. You could run two of either on your current PSU.
I am getting an AMD 7850 tomorrow and for the price/performance/power ratio it is awesome. What is your budget?
I was looking into that as well because of its price and great reviews. My price would be about $250-$300.
If I get the extremely good cards like GTX 670-680 or Radeon 7970, it will be bottlenecked due to my CPU.
It might bottleneck in some games but it would be ready to put into a new system at some point if you are thinking of upgrading your motherboard, cpu later on.
What speed it your phenom 2? Is it overclocked?
I run an MSI 7850 and used to have it paired with a 965be clocked to 4ghz. Its a great gpu and probably the best bang for your buck in the industry right now. I run it on a Xeon now, but I love it and recommend the MSI for great overclocking and a very quiet, cool heatsink. Sapphire is another good brand that I have used in the past.