The PSU has more than enough juice (but in theory it still can be the PSU if it's failing). I ran ASUS P5B-E, E6320 @2.8GHz, 2GB DDR2-800, DVD-RW, HDD and 8800GT with noname 350W and it was just fine.
Remember that even the most efficient PSUs have only around 80% efficiency, meaning that with 550W PSU you will be losing minimum 110W, even when totally idle.
With that 650W/720W it would be 130W/144W lossage... Ouch.
