If you can afford the 350W,, do it. The 300 may work,, board has a lot of features, and needs QUALITY PS (and mem). I'm using a Enermax 350 whisper and have the temp sensor taped on top of the supply. I very rarely hear the PS fan kick in. I have 4 case fans (low RPM) and a 34CFM-80mm on the cpu, with raid 0, ATI8500, and 1 PCI modem. All fans execpt for the cpu are running though a Digidoc5 so I don't tax the mobo fan v-rail.
I can overclock the heck out of it and burn a cd at the same time, no problem. If your going to fill the board with a lot of PCI stuff (not recomended due to IQR conf. but can be worked around) then go the bigger unit. JMHO