Will a PSw/20 pin work on mobo w/24 pins?

Will a power supply with a 20 pin connector properly power a board that accepts 24 pins?

Yes (probably). The extra four pins are just another 3.3, 5 and 12 volt lines (and a ground). This allows an extra 6 amps of current per line, but unless you've got some funky setup it's unlikely you're going to be drawing that much power over the main connector anyway (bearing in mind the CPU draws additional power over the auxiliary connector, and higher-power graphics cards take one or two four-pin Molex connectors for supplementary power).

Yeah, but you have to rewire it into the fdd header

black wires go on inside lol/

>>3
No. You just leave the four unused pins unused. No rewiring required. No adapter required (an adapter just adds further resistance to the circuit). They're designed to be forward- and backward-compatible (you can plug a 24-pin supply into a 20-pin board and vice-versa, provided the power demands aren't too high in the latter case). I'd only start worrying if the graphics card is pulling the maximum power over the PCI-E bus (75W) without using a supplementary power connector, and even then a 20-pin connector is quite capable of pushing 250W total to the board alone (and you can take the demands of the CPU off that with the supplementary CPU power connector doing most of the work).