Take your power supply with you to a computer shop so you can match it with a more powerful one.  The new one will need at least the same power cables you were using and the same number of them.  The same power plug hole on the back as your old one (and in the same location, top right, lower left, etc).  Some have power switch on the back which needs to be in the same location also.  Otherwise, you are like me and cutting holes in the back of your computer case so the new power supply can be plugged into the wall.  Also match the power supply fan/vents with the old one.  Bigger power supplies may have fans on the outside instead of inside their case.  Remember how much room you have around your old power supply so the new one will still fit in the computer case without hitting the back of your DVD drive or rub against your CPU fan/heatsink.

Your case won't need extra fans.  The fan with the new power supply will be enough cooling.  If you feel air (warm or cold) blowing against your hand when put behind your computer, it's doing its job.  Remember, computers like only one of four elementals -- Earth, wind, fire, water.  Computers hate dirt, hate liquids, hate heat, but love moving air.  If your computer is in a desk cabinet, take it out.  Heat and dirt collect in there.  But you know this probably.

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