Need a new Power supply?

It would help, if you would like for the opportunity to be there for more people to answer

your question, for you to give more detail as was suggested that you do...such as which computer do you have and why the other PSU's won't fit...

well you need to open the tower to look at it

if it's a dell or hp/Compaq or IBM 99% of the time you can not change to aftermarket PSU

especially dell

but as a rule of thumb the majority of PSU are about 300W

if you can buy new depending on what you want to do (plug in device etc)

are you looking to go sli/Crossfire? look for 600W -1000W (1Kw)

or more

if you are keeping it stock just want to get a better power supply unit

look at a quality made 400-550W range

better to have more power and not use it then to have not enough power when you really need it