Wired and wireless G here on a network with 7 (Linux) PCs and a (dual boot - it can run Vista if it has to but is usually Linux) laptop. There is a faster Wireless N std.

Wireless is to enable the laptop to roam and has also has enabled other family members to use hand held devices when they have visited.

For the wired (which I prefer for speed and reliability - but cabling can be a pain - I actually ran a couple of legs outside the house to simplify my task), we are on gigabit and I do notice the difference between that and the previous 100-T when using mythtv and there are tv recordings etc. moving around.

All of the above are faster than our (typically) 1.5Mb Internet connection and I would guess which ever way the OP goes, the Internet would be the first bottleneck encountered (if encountered).

I've not tried the mains solution and would avoid it here because X10 (home automation) already goes down the power lines I've have read claims that the two can co-exist but I'm not entirely convinced, and our X10 is only just on the fringes of working in places as it is. Thinking of possible interference with this route. It might be OK (I haven't a clue) but I also noticed (when looking at one for our roof space) that certain types of pest deterrents can also send signals down the mains...