Like Michael said, 20GB is way too small. 60GB should be your minimum size especially considering that they often are going on sale now for $1 per GB or less.
As far as power, I recently measured the DH67BL w/ Core i5-2500k, NVIDIA GTX 560 Ti, 1 SSD, 1 7200RPM drive using a 750W PSU at 59W idle. You should be able to trounce that with a more appropriately sized PSU and just using the integrated Core i5 IPG.
