I'm giving you best answer, though the PSU did prove not to be up to the task. After a couple of days the NAS box started rebooting with the power led flashing for a bit. it was slow at first, then started becoming more frequent. Finally I pulled the 300W PSU out and placed on old 380Watt in there. I wasn't sure if it would work since the mobo has a 24pin connecter and the old PSU only had 20 pins.
I did some research and discovered the extra pins were to provide more power for power-hungry system, and were entirely necessary. Its been running fine with the old PSU, but ordered a 620Watt one since I'll be adding 4 more drives , and probably an 8-port SATAIII card and a dual-port gigabit ethernet card( so more power will be needed ) .