I think my PSU (OCZ Powerstream 520w) actually has a both 24pin & 20pin out. You can detatch the extra 4 pin from the main 20 pin or clip it on for a total of 24pin. I also bought an extension cable, 24pin (female) to 24pin (male) which should do well.
For any of you that have this case:
The case is a full size tower. The PSU as at the very top of the case. Since the motherboard is upside down, the power for the mobo is at the very bottom. After I looked at the massiveness of the case, there is no way in hell that the stock cable from the power supply will reach the motherboard. It would be an incredibly tight stretch, not to mention the amount of bending that would be required which could even damage the cable if not done properly. I suggest getting an extension even if you are unsure. The one I bought was a boring $10.