I believe the 6604-G is a part of the relatively new Ultegra SL line--a half-step up from Ultegra to Dura Ace. There's information on bikeforums and elsewhere about how SL compares to regular Ultegra.
As for compatibility, I imagine they are all compatible--though I'm not certain. Look on http://www.shimano.com.
Sounds like you intend to run a triple crankset with just the inner ring at 24T? I'd be concerned about the cross-chaining between the chainring and smaller cogs. I'd be inclined to go for an older system like aquare-taper, ISIS, or Octalink, so you can swap in a longer axle and move the crank out a bit resulting in the chainring better centered relative to the cassette.
Edit: on second thought, the system may be OK. I just checked my road bike with 2-piece triple crank and 10-speed cassette. The inner chainring lines up pretty well with the 4th largest (lowest) cog, so it's not too far off from center and maybe you'll be using the larger cogs more for climbing anyway.
Last edited by JiveTurkey; 06-20-08 at 11:50 AM.
Originally Posted by slopvehicle
Not wearing a helmet makes me more aware of my surroundings. I find myself anticipating the hardness of concrete 50 or 100 feet in front of me, it's almost a zen-like connection between my face and the pavement.