If this is your PSU: https://www.facebook.com/237086263080655/photos/a.26421...
...it appears to have a small 15A (+12V) rail. Not exactly a 'real' 450W PSU. That's more in the 250W category. So you will be limited to something very mild unless you upgrade to a better PSU.
Even the HD 6790 is too much for that PSU. I would not go more than a R7-240 with that PSU. If that is not your PSU, please look on the nameplate and read the +12V rating.
You're not going to find a card that you'll be happy with as long as you have that weak PSU. By weak, I mean the all-important +12V rail. That's where manufacturers of cheap PSUs cut back so they can call their PSUs "450W" and get away with it.
Something like the R7-240 will still game fine at 1440x900 with reduced settings. Check out some videos: https://www.youtube.com/watch?v=JkfFWhuecr8
But that's about the best I can recommend until you upgrade the PSU.