your brother seems to have the best advice for you! I have never heard of such a fairy tale as a motherboard being limited to a 300 watt PSU >>>> thats laughable!
any PSU under $100.00 is not going to meet your needs for long
I saw a 700 watt OCZ or Silverstone ??????? at tigerdirect that was $94.00 after rebate that is a smoking deal
BTW: too much PSU will never hurt you! they only put out what the system needs. The problem come when the system needs more than the PSU can put out, then you start the frying process.