I'm a big fan of the hardware approach. I started by software boxing and there are (solvable but in my case annoying) issues with capitol city lag, key press fowarding lag, etc that you completely avoid by going full-hardware. Also I really didn't like relying on a piece of software for key multiplexing.

IMO your secondary machines don't have to be all that uber. I would recommend more than 2g ram; 3 of my slave machines all have 4gb, the last has only 2, and this last is dramatically slower to zone into Dalaran on login or hearthing. This has everything to do with system ram available as two of my slave machines are Mac Minis with piss-poor video cards and laptop hard drives, but 4gb system memory, and they zone in just fine. Your main machine will of course be where you spend the money on video card etc.

Other than that I didn't really overthink it much, and haven't had any problems. Synergy is the only software in my mix (for easy clicking around). I use a Logitech wireless keyboard with 5 receivers all set to the same channel. Profit.