The SU is 450w on this machine
Not all PSU's are created equal, but if a decent brand name, 450W should be fine (i was using a Seasonic G-450w (Gold rated) until a power failure spike killed it).
The biggest concern you may have is general 'bottle-necking' in your overall system, so even getting a better GPU (like that R9 270x) might not give as much performance boost as it should.
I have an old system (was my main gaming system until a few years back) that also had a 9600GT as the main GPU, and the rest of that system was very old (AMD FX-90 socket 939, AX8 mobo, Windows XP etc), so much so i needed a brand new build to see proper FPS improvement in my games. A better GPU would not have helped that much overall. So keep the age of the rest of your system in mind also.