I would say, use it till it burns up, then only replace the one that has gone bad,

Its nice because the cap that goes bad has a built in detection system, (it will look crispy)

I changed a chard Tantalum Cap on the LB my powerbook 170 mainboard, the cap was on the underside of the LB, very close to the power plug. This was one of those (does not work 99 cents plus shipping ebay finds)

Also real quick, make sure to use a multimeter, and make sure your power supply is outputting the proper voltages.