The link below might give you some peace of mind:http://www.computer-aid.com.au/blog/2007/06/04/20-pin-vs-24-pin-power-supply-and-pci-e/
There are alot of power supplies that have a 24 pin connector with a detachable 4 pin connector leaving a 20 pin connector.
These are designed to accommodate older and newer Motherboards.
Older motherboards didn't require as much power input as newer ones that have the ability to... let's say run a few add-in cards, such as two graphics adapters.
If your system isn't an energy hog you will be fine... on the other hand if you have some extra hardware peripherals connected to the motherboards be concerned.