300 watts is more than enough for most any desktop system.
However not all 300 watt supplies are 300 watts. Further
problems are demonstrated by this website:
http://firingsquad.gamers.com/guides/power/default.asp
Look at systems designed by engineers. Many are below or
just above 200 watts. But then those 200 watt supplies truly
can provide 200 watts. Notice the problem cited by Tom's
hardware:
http://www6.tomshardware.com/howto/02q4/021021/powersupplies-15.html
Time and time again, our lab measurements were unable to
verify the output figures represented on the model
identification sticker. And how, exactly, is a computer
purchaser supposed to check the output of a power supply?
Really quite simple to identify a supply that cannot provide
the load. Install the 300 watt supply and run multiple
programs that execute all peripherals simultaneously. Using a
3.5 digit multimeter, critical voltages (3.3, 5 & 12 on
orange, yellow, and red wires) are monitored. If not large
enough, then when supply approaches its upper limit, voltages
suddenly start to drop. Not enough to cause problems, but
enough that it is clear: the last peripheral starting up has
taken power supply to its upper limit as indicated by a sudden
increase in voltage loss.
A simple test, done in minutes, that can answer your
questions since product labels cannot be trusted.
Most common among clone builders is the use of power
supplies that are even missing essential functions - common
among supplies sell for less than $40. 'Self destructive'
supplies are demonstrated in that above Tom's Hardware site.
All outputs for any acceptable supply can even be shorted -
and still no damaged. But in Tom's, some supplies were even
damaged by a less than maximum load!
Many supplies are that inferior as to self destruct. Many
clone builders use the 'we don't need to know' solution called
"More Power". Instead recommend 400 and 500 watt power
supplies rather than fixing the problem: a defective vendor
who sell product only on one specification - price.
300 watts should be more than enough *IF* supply is from a
responsible vendor. Responsible means he also provided a long
list of numerical specifications because essential functions
were inside the supply. If not sure, then simply perform the
above 3.5 digit multimeter test. Learn yourself if supply is
sufficient. Properly sized power supplies go to 100% load
without a sudden voltage decrease.
BTW, why the voltage drop? Because if too much power is
demanded, then the power supply goes into current limiting - a
self protection function. Another reason why supply outputs
can be completely shorted and supply is not damaged - as was a
defacto standard even 30 years ago and require by Intel
specs.