What nvidia recommends is total system load..and they overcompensate for crappy psu's. The neo 480 has a total of 33 amps...plenty for a system with a single gpu.
Even with a GTX 260, his max system load will be around 300 watts. My folding rig with dual gpu's and a dual core is running off a 380watt earthwatts, and the load on it is around 300watts with 24/7 usage.
Interesting... It's possible that the 3dmark06 score I got was accurate. After looking at the other scores' processor settings nearly all scores that are higher have understandably higher processor clocks.
You could oc the cpu to 4ghz and still be well within the output range of the psu. Only reason to get a new psu, is if you wanted to run dual GTX's or a GTX 295.
And yes, 2.5ghz is hurting your 3dmark score. With the chip at 4ghz, it would be way higher.