That PSU is terrible, only 15 amps on the +12V rail, so it's really a 200 Watt PSU at best. You're going to need a new power supply for any gaming capable GPU, so you'll have to upgrade it too. Either save up an extra 50 bucks to get a good quality 450 Watt PSU as well as the 6870, or take 50 bucks out of your GPU budget to get a better PSU.
I wouldn't recommend that one. The CoolerMaster Extreme Power Plus line is of rather poor quality. Their voltages tend not to be stable when pushed to their upper limits, potentially causing damage to your components. The only CoolerMaster units that are really any good are the Silent Pro Gold line, and they start at 700 Watts.
Antec, Seasonic, Corsair, and XFX are among the better brands, and they will have good quality units available in the 450 to 500 Watt range.