You WILL need to change your PSU (or at least it's 99% likely) - the 7100 you have atm uses very very little power, so you are effectively asking your PSU for another 100w - On a pre-built corners are cut whereever possible and so the smallest PSU you can get away with is used.
But prebuilts often use weird PSU sizes and sometimes even built them into the case. On my current PC the PSU is held in with 4 screws from the back, but on my last Dell it was welded into the case
My rig had a 250w PSU when I bought it. It was a Packard Bell iMedia 1509/1508 and I went from that to a 550w Q-TEC in the standard case (:really and now have a 600w OCZ one in this custom case, both fitted without problems.