I think you need an HD video card too. My friend just had to get one for his machine so he could edit HD movies. I’d assume you’d need one to play them, too.
Depending on your OS, you may need some or all of the following: Video card with approved output port, video cable of approved format, and an approved display.
In this case “approved” means “blessed by the content consortia to actually display their movies at high def”. I don’t know the specifics of which OSes (I’m pretty sure about Vista, don’t know about XP) and which cards or monitors have these restrictions, but make sure you find out beforehand, or you could be left with half an HD solution.
I believe Vista’s currently the only OS with that sort of requirement. Current hardware will generally be labeled Vista-ready if it’s blessed by the [del]cartel[/del] consortium.