I have a 12vATX 300W PSU and I am looking to buy a cheap (but new) motherboard for it, now the cheapest ones at TigerDirect are uATX and micro ATX. My PSU comes with the option 20 or 24 pin PSU to motherboard hook up. So is that all I need for it to be interchangeable between boards or do I need to get a different PSU for use with the smaller form factors?
no. atx boards should work fine with your power supply if it has both the 20 pin and 24 pin connections (usually an addon to the regular 20 pin).