I've read that you should use at about half the capacitance when replacing a low ESR for a solid cap in VRMs, and mantain the capacitance everywhere else. You should be able to dicern between a low ESR capacitor and a "normal" one by reading the family and brand written in the capacitor itself.

It depends on the hardware. If you list the specific motherboards you want to recap, it would help in giving you advice.

One thing you must keep in mind is that solid caps or polymers are very fast, have very low ESR, and will be a huge draw on all the capacitors behind them, i.e. the power supply will get hit harder. Sometimes it's best to simply replace the capacitors supplying power to the CPU and RAM with polymers, and leave everything else alone or replace them with regular electrolytics. But it's best if you have a high-quality power supply or recap an average or better power supply with quality electrolytics.

The previous suggestions about halving the capacitance are true, depending on where the capacitors go. The VRM for the CPU is not about supplying huge amounts of power, it's just about low ESR, speed and reducing ripple, i.e. smoothing power delivery--this also holds true for the capacitors handling the RAM.

If you're dealing with a single-core Pentium 4, then polymers are really only necessary around the CPU, maybe RAM if you're trying to maximize overclocking. I have recapped an entire Asus P4PE-L which was running a 3.2GHz P4 with polymers and it caused the system to be noticeably quicker and snappier in everything it did.

Pentium III's and earlier don't benefit from polymers in most peoples experience as the arsenal of low-ESR electrolytics already do the job with lots of leeway.

Here's a great, hands-on article on the subject of recapping with polys