the upcoming boards should be compatible with either 20pin or 24pin PSUs...besides the pci-e cards will all have extra power connectors.
The question is rather whether the old PSU is sufficient to provide the appropriate wattage...eg. you should have at least a 460W good brand PSU (Enermax, Antec, Fortron etc.). 24pin would be better in the long run (eg. a nice 24pin PSU with dual 12V rails, eg. every rail 18A). So you cant go wrong in getting a new 24pin one - but this doesnt mean that an older 20pin one is automatically insufficent.