you can look at your psu inside your case to make sure it is an atx as well. Look HERE
See on the label how it says ATX 12V in red at the top? If your PSU says ATX on it then any ATX will fit just fine. How much do you want to spend on a new PSU? I agree with everyone that you should definitely upgrade it. Look for one with dual 12V rails, this increases system stability across different devices.
I like FSP or Fortron PSUs like these and I also like ANTEC Truepower series.
One last piece of advice = stay away from the budget PSU. You do not want bad power going to all of your expensive components. Spend a little more, get one that lasts and the rest of your system will last longer too.
i reccomend an antec smartpower. the 400w is $55, the 450 is $60, and the 500 is $75 (also features modular cabling). i use smartpowers all the time and they are always reliable, powerful, and quiet.