I would think the first thing to do is find out which local stations are broadcasting in HD, and what HD services your local cable company offers.

HDTV ON The Air (You'll find other good info at this web site also)

Then, if you already have Directv, examine what they have to offer.
DIRECTV HD

I would think they offer more than you could get off the air.

If you're really HD crazy check out Voom

An alternative might be WOW Cable TV provider if it's offered in your area.

Jack

"People generally quarrel because they cannot argue." - G. K. Chesterton