You can run gigabit ethernet over CAT5e (and probably CAT5 if you wanted to) without any issues. Mostly, it comes down to the termination and quality of the end connectors; whether you terminate the wires correctly. CAT5e is cheaper than CAT6 as well, and won't be replaced any time soon, esp. given most harddrives (sata/ide, in a non-raid array) don't have fast enough I/O to meet the maximum transfer rate of gige.
I've been interested in the MoCA stuff, but don't have a use for it now, with CAT5e to every room. I've heard a lot of back and forth on Powerline networking. Generally I hear it can be problematic due to interference issues. Back in the day, I had good luck with HPNA cards in two computers. 10Mbit OMG crazy speeds (this was 1996 so that was pretty fast). From what I've seen, they're now 100Mbit standards for HPNA.
Some day I'll set up a htpc. Pretty cool that the Atom can power all that without a lot of lag. Must be quiet as well (a necessity in my opinion for a htpc).