My mini-itx board has a 55 watt power supply. Yup, you read that right. 55 watts. It has a 7200 rpm hard drive, an nVidia GeForce 4 card, a USB remote control, and a USB Playstation2 controller adapter (with 2 controllers attached), and runs fine. Even though that motherboard will suck up a lot more power than the mini-itx, you should be able to run 2 hard drives, a cd/dvd-rom/rw drive, a handful of USB devices, and a couple of extra PCI cards without even taxing that 300 watt power supply.