I wouldn't worry about power - we've had some good discussions here before, but basically the deal is that even at high volume levels most components don't draw their "nameplate" rated electrical load - I've got a Model 750 amp (Outlaw's original amp, 165Wx5), two Model 200's, a 32" CRT, an LFM-1, a 990, three DVD players, and a cable box on the same circuit and have no problems.

Does the Sony amp have pre-amp outputs? If so, adding an amp like the 7125 would be a nice upgrade that would also allow you to upgrade to a new processor later. The 7125 alone may not help with the weak surround sound, though, since I doubt the surrounds are being starved of power. I'd try recalibrating as a first step (won't cost you much, just an SPL meter if you don't already have one), but it may take an upgrade at the receiver to really get things rolling in that regard.