See the thing is, I'd say you want at least 450 watts just to be safe, and for future upgrade ability (SLI, etc.)
With the exception of future water cooling or SLI, I don't see to much of a need for future proofing a PSU. Things are only getting more efficient, something that needs 500w today will only need 400w tomorrow.
The only thing I can see is new connectors, which would make the most expensive 1000w + today obsolete.
http://extreme.outervision.com/psucalculatorlite.jsp
q6600@3.6 w/ 1.5v, 2 sata hdd's, dvd/cdr combo, 3 120mm fans, 8800GT(power hungry) = 358w full load.