I use the same type pulleys as dd. I bought mine at a sailing shop, about $12 each. I use the 1/4 rope for both the tree and pack lifting rope. My set-up is a little different, as I use the Modification Azalea recommended. It just allows you to keep pack closer to one of the two tree's to eliminate almost all of the line sag.
I'm thinking of trying to use some of that 1/8 braided Dacron rope they sell at Cooke or Piragis outdoor. Holds 450 lbs and weighs .05 oz for 80 ft. Has reflective cord in it, so should make for some cool night time effects.  Supposedly is stiff enough for easy knot tying. I figure that for tree rope and keep the 1/4 for pack haul rope. Anyone have any experience with  the Dacron guy line rope?  Overkill weight wise, but would lighten the kit significantly IF practical?

  I use the two rope method for the tree hanging rope, seems easier too me as well, and allows for Azalea's modification.
  I remember Mad Mat's recommendation about using a lighter heave line in case you snag, and had that in mind when I contemplated the switch to the Dacron line.  I still think I'll keep the 1/4" for the haul rope, easier on the hands.  If you don't have the 3 pulley set-up, and your hoisting a heavy food pack, you owe yourself too make the switch. Even a small person, with minimal effort can easily lift 90# with ease.
Thanks for info on Dacron kite line

This is 1 pound almost exactly (at least when they are dry ):
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These are the pulleys I think I used and I thought they were expensive but I knew what a pain cast pulleys were. Back story: We used 69 cent cast pulleys where I worked specifically because they did NOT spin very well so our counterbalance was not critical. The guy that tried these quickly switched back to cast - so I got freebies!:
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The top rope is poly and the pulling rope is nylon - 50' of each I think. The big rope is nice on the hands, never tangles and most importantly, it fits the pulleys I have. If I were to switch to smaller dia. rope, I'd also need new pulleys since it could easily foul and lock up between the wheel thingy and the frame when using rope the pulley wasn't designed for. Separating the top two keeps the ropes apart so there's no drag although I invariably need to raise my pack a foot and spin it to separate them at the start.

I made my setup in my basement one winter and as luck would have it, haven't changed a thing since. Even used it today to hoist my snow blower to it's summer home atop the garage rafters. (Winter is now officially over eh? Whoo hoo!) Anyway that sucker is a lot heavier than a foodpack for six.

The other thing to keep in mind is you don't want the top rope tight. At least I don't. I want some play so that when the trees start swaying in a storm my pack doesn't come crashing down. There's got to be serious force involved with that so I choose to avoid it with slack.