Before you run to replace the old one, if you feel confident, power off your machine and unplug it from the wall. wait 5-10 minutes for things to cool down if its been running, then open the case and remove the PSU (you'll have do this anyways to replace it) and see if there is dust visible from the outside. DO NOT OPEN THE PSU! the power supply has large caps inside it that can store a charge for about 45-60 minutes even with the bleeder resistors. After waiting about 1 hour open the disconnected PSU and check for dust or wires that have come un-tied and are hitting the fan.
To answer your question more directly it will probably need a 250w ATX power supply but like the above posts, it depends on your hardware.