Just to throw out there OP, HD video is actually a rather easy thing for many computers that were made even almost 10 years ago. I remember I had an Athlon XP 2500+ that could, though barely, run 720p video. Considering the performance jump between that old dinosaur and what you have, I have no issue seeing even the onboard GPU (as FascistNation mentioned) being able to handle 1080p.
Also the bare minimum, we'll say, for HD content, as per Blu-Ray specification which is much more than what you find elsewhere, is 4.5MB/s (36Mbps). PCI has a maximum throughput of 133MB/s assuming no other devices are on it.
I agree with you but additional Video card makes Videos looks sharper specially good for HD. Go for it you are videophile.