[BrianG]

Just a warning in advance; replacing caps on a motherboard is a ROYAL PAIN! There are several layers of copper, some of which are ground/power planes, so the board tends to wick away soldering iron heat fast. There is a fine line where the heat/power needed to remove caps is pretty close to the amount that will damage nearby components. To help minimize heat build-up, work on different sections between each cap removal/replacement.

As far as the capacitance/voltage rating: Very few caps (if any) are used for timing on a motherboard. The frequencies/duty-cycles needed are very precise and caps just aren't accurate or stable enough as they tend to "drift". Besides, timing caps are generally not electrolytic but are ceramic, tantalum, etc. So I would be 99% certain that all those electrolytic caps are for filtering.

Like was said, going up higher in voltage or capacitance shouldn't cause any problems, but they do get physically larger/taller which may interfere with other components, heatsinks, etc. So do try to get close to the same values, but do not go lower. No matter what, make sure they are low-ESR type (105ºC rating).

Good luck!