You wantto look at the power supply and see if it's an ATX style power supply , if it is then you can upgrade to an ATX psu. If it's another style like miocro atx or TFX then you have to look for those style psu's.
The ATX is the most widely used power supply and you will have no trouble finding one and you may consider a 400w since it's not that much more.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371046
Antec High Current Gamer Series HCG-400 400W ATX12V v2.3 / EPS12V v2.91 80 PLUS BRONZE Certified Active PFC Power Supply
$59.99 and a $40 rebate makes the final price $19.99.
Something like this is too good a deal to pass up , a $60 psu with a $40 rebate so you end up paying $20 for a 400w psu.