Really, if you need a new GPU so badly, your going to need a new PSU, GPU (derp), and that's about it.
I had 370 watts for awhile on my Geforce 210 by EVGA, but that didn't do the trick, I got a Corsair CX430 and that was rated at 430 watts.
That helped me in MANY ways, I could play TDU at full res, and It boosted some FPS on other games.
I suggest looking at your power supply and read off the wattage, if it is <300, get a new one.