Hello Folks:
I know this is an older thread but I thought I'd add my thoughts, on my previous layout, which had a life of about seven years, I found that eventually I had to bond all the rail joints as the joiners themselves fill up with gunk and don't fully conduct, the gold plated Peco joiners were very good, bet they're not made any more! Spraying with a good switchcleaner periodically, Servisol is good, (not WD40 it's an oil), does help but the older the layout got I had to put more bonds in, a gas soldering iron is good for the work, and the wire can be pretty thin.
As regards track power, Tom is right on the money, the thicker the copper core the less resistance over longer cable runs, there is nothing to prevent you using thinner cable from the track itself to a lineside junction box, but the long lengths should be at least 2.5mm.
Lighting feeds can be pretty thin, especially with LEDs. For point motor feeds I used recycled quad microphone cable, I use the screen as common and I can feed two motors from one run, however I do use a good cap discharge unit off a twenty four volt supply, so there's plenty of "thwack"!It will happily fire two motors on a crossover. Computer cable could be used for points or lighting I would say, although some of the cores are thin, so I really wouldn't recommend it for track power.
I hope this is useful.