With 460 watts it depends a little if its brand name or not, you should be able to wring out 150 watts for the video card though and the 6870 is 151 watts TDP.
I wood and did choose an Asus GTX550 Ti, to be installed soon, after I replace my 350 Watt supply with a 600 Watt supply. At least 400 Watts is recomended for the GTX550 Ti.
The power supply is a little close for my comfort zone but that being said I would go with the GTX550 and seriously think about a power supply upgrade.