In article ,
lid says...
NEVER trust the advice of a salesman! They are paid to sell, so always make
independent enquiries! She may however be right, but the only way to find
out is to try it with the 300w PSU and replace it if need be.

As opposed to asking complete strangers here on USENET?
(j/k... kinda)

To answer the O.P., getting a new power supply from a
reputable vendor (Zalman, Antec, and a few others) is
always a good idea. Especially if you get one with the
side intake fan to help pull heat away from the CPU area
and out the back of the unit.