Ok, yeah, I didn't look at the power supply at all.
# PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
# 450 Watt or greater power supply with 75 Watt 6-pin PCI Express® power connector recommended (550 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode)
That power supply is pretty limiting. You don't really have a whole lot of options with a 300 W PSU. None of the current (5nnn series) ATI cards suggest less than a 400 W PSU. And they're generally less power-hungry than the previous generation. Perhaps there's an nVidia card, or something from an earlier generation available.