However, they recommend a 300 watt PSU...not sure why.
It's because they have no control over the quality of the PSU you might buy and don't know what the rest of your components might be. So, they all overstate the requirement rather than dealing with false customer returns when you plug in the card and the PC doesn't boot correctly.
The lowest end GPUs from ATI and Nvidia weren't good enough for HTPC because they didn't have enough processing power for some formats/de-noising/cadence detection, etc. My recommendation is to ask the guys over at missing remote
as they live/breath this stuff.