300 watts is up to the task in most cases. However many 300
watt power supplies are sold on price rather than on value and
abilities. Many 300 watt supplies may not even supply 300
watts. And many who repair computers don't even know why
failure happens - they just use the Tim Allen concept of 'more
power' and assume that solved the problem. This demonstrates
why the problem may be even more complex:
http://firingsquad.gamers.com/guides/power/default.asp

Get the meter and stop guessing. If power is a problem,
then the meter will show it up front and immediate. You can
spend your life wildly trying this and that. Or save time,
read the numbers, and isolate the problem - immediate and
obvious.

Chances are if the system was working just fine previously
and if nothing was added, then 300 watts was more than
sufficient. Currently, there is not even one good, reliable,
known number to work with - to provide a solution. No number
means junk science. Get the meter. See if the power supply
really is a problem - rather than guessing.

Nick wrote:
ok I'll get rid of the old 4GB HDD just in case

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Nick