It might be worth having a look on the ultralight hiking pages, I am sure that the guys on hammockforums.com have lots of opinions

For guylines, for instance...
http://www.hammockforums.net/forum/showthread.php?t=14262
http://www.hammockforums.net/forum/showthread.php?t=2660
I use Marlow 2mm Excel Pro (sailing line), 16plait polyester low stretch for guylines. Light, strong, good gripping surface for adjustable tension knots, and bright enough that it doesn't get tripped over too much.
I have used different stuff for the ridgeline but there is much to recommend one of the 3mm 16 plait sailing lines. The only problem I can see with these lines, so far, has been that the lack of stretch means you have to be pretty good with your tensioning knots. With a stretchy line, like 550 cord, you can allow your tension knot to slip by an inch or so and you still have tension on the line. Non-stretch lines aren't forgiving of slips in the knot tying, they are either really taut, or not taut at all. This lack of stretch can also mean that you can wake up with a slightly less tight tarp if it rains during the night, the lines can't take up the slack when the tarp material stretches.
There is also an arguement that non-stretch guylines can impart a lot of load to tarp tie out points in strong wind.
http://www.jacksrbetter.com tarp uses short pieces of surgical tubing mounted in the lines to prevent over-tensioning, give a little shock absorbtion, and take up slack if the tarp stretches.