Indeed wattage means nothing on comp PSUs.
Wattage is volts multiplied by amps. Then when you add all of the watts together you have the total watts.
The xbox PSU only has a large 16.5 12 volt rail with a 5 volt 1 amp standby rail.
A computer PSU has many more it has a 3.3 volt rail, 5 volt rail, 5 volt standby rail, -12 volt rail, a 12 volt rail, and on some other supplies an additional 12 volt rail.
As you can see there with all of those rails even if the 12 volt rail was lower then the xboxes 16.5 amps the other rails could compensate and easily affect the total wattage rating.
I'm using the Antec Truepower 2.0 500W supply you can find them on e-bay for around 30 to 50 bucks.
This PSU powers my comp its got 17 amps on one of the 12 volt rails and 19 amps on the other and its also got 2 fans the intake fan runs when the supply is on and an additional exhaust fan powers up when the PSU temps begin to rise beyond when fan 1 can handle. The fans on this supply are whisper quiet. Much quieter then those leaf blowers on the 360 anyways
. Thats next on my list.
I haven't received my cable yet but I took out my computer supply and temporarily spliced it into the xbox for testing and it worked great.
Antec stopped making this supply because stupid customers returned them thinking that the rear fan wasn't working it was so bad actually they had to put a piece of paper on the back explaining it
I would stay away from cheap Chinese brands since they have no problem outright lying on the labels about the PSUs amperage on the rails and they only way you will find this sad fact out is when your PSU turns into a smoking paperweight.
I have an @-Power supply here which claims 450 watts when I opened the supply and checked the heatsinks for heat dissipation and the rectifiers (one of the rectifiers were 2 discrete diodes clamped to the heatsink) rather then a standard TO-220 package from my calculations that supply was 250 watts at best.
Stick with known brands your 360 is worth it.