The Manfrotto 501 / 503 heads are not really up to the task for HD. Don't take my word for it - search the sticks forum on this site. The 504 might be different - the Chris Soucy comparison test (in progress) will be interesting. The best choices would be true fluid heads from sources such as Sachtler or Vinten.

all the manfrotto / bogen 500 series heads are crap. they are cheap, and the will not do what you want.

personally I have the predecessor to the vinten vision blue, the vision 6 head. its a decent setup and you can pick one up for $1k or less used. only single stage legs, but for more basic shooting its ok. I finally got myself a set of baby legs to go with it. if you can get 2 stage legs, do it. you'll have no regrets.

as for the gitzo tube legs, they are such a PITA to work with... you can to turn each leg lock, its slow, clumsy, and if you don't get it 100% it will slide. then most folks have the gitzo without a spreader so you will be messing around to even up the set of the legs. if you are on a slippery surface, they can move. I'm sure some one will tell you how great they are, but having used them in real run & gun news / doc shooting, they suck to use more then most pro legs out there.

get something with a single motion leg lock, where you know its fast, its locked.

I tried a Sachtler DV1 head on ENG 75/ 2D sticks recently, and it was perfect for the 7D with a medium sized zoom lens. The configuration is really light weight but stable, and the fluid head works perfectly for a lightweight camera.
I don't think they make the DV1 head anymore, but any of the small heads for DSLR or HDV cameras should be fine.

Every Manfrotto tripod I ever used was mediocre at best, some were horrible. Sachtler is another league, their products are more expensive, but they're worth the money.
I mean you can use a good tripod forever, so why not invest in a really good one?

my guess
 

I have the single stage Miller DS-20. The head is amazing and the legs have 3 knobs you turn and the legs fly out and you tighten. I have the mid spreader which is what you want and not the floor spreader.

I would get carbon legs to make it lighter.

My two stage Miller is a bit more of a pain to open and close and for documentary you might not need the extra height.