These types of (motherboard) bulk power caps do not have to be 6.3V nor 1500 mfd.
They DO have to be higher than the PSU's 5V rail in some subcircuit placements, so 6.3V is common, or 10V, 16V would also work. If you can trace the circuit and use a multimeter to measure you often find they're even below 3V potential- but at this point with the board down that's harder to do. Point is, the role of these isn't such that specific values are critical for voltage or mfd.
1500 mfd, a nice target but 1200 mfd would probably do fine, but bigger being even better. If they were 10mm diameter, far higher mfd would be even better- just not extreme, you dont' want the system power supply driving into a 40,000 mfd capacitive load on system power-up.
Of particlar important in switching power circuitry roles, is that they be VERY low ESR. Not just "low ESR"... that may be why they failed in the first place (unless they were actually defective). Rather, as low an ESR as the budget and space allows, should be the goal.
Towards these ends (and considering that the original part is an 8mm diameter), among the best replacement options would be the tallest 8mm diameter 6.3V Panasonic FM series that Digikey sells. In other words,
these.
This is coming from someone who replaces hundreds of such caps a year... the mfd really isn't as important as the ESR value, they're larger mfd value is simply a function of the larger can size chosen to achieve the lower ESR while still retaining "enough" capacitance far beyond what a ceramic would, and at lower price-point than typical hybrid compositions.
