I hate to drop this in, but it is not always about throwing top end hardware at it.
My setup has 3 machines.
Machine 1 with primary wow set to makes graphics
$150 from a best buy open box
2.6ghz dual core
2 gigs of ddr2 memory
8900 gt video card

Machine 2 with 2 slaves set at at lowest graphics
3 year old Aleinware laptop
3.6ghz hyperthread processor
2 gigs of memory
6500gt mobile video card

Machine 3 with 3 slaves set at lowest graphics
$150 from a best buy open box
2.2ghz dual core
1.5 gigs of ddr2 memory
8900 gt video card

All three machines are tied into 24" gateway monitors, avast anti-virus, windows firewall active, octupus for km sharing and keyclone. Machine 2 has ventrillo
Machine 1 is not fps limited, machine 2 and 3 have a max fps of 15 for both active and background.
Machine 1 hits around 25 fps in dalaran, machine 2 hits around 10 fps on both characters and machine 3 sits at 15 fps.
Grand total was $300 each for machine 1 and 3, $2.5k for machine 2, but that was 3 years ago and depreciated value puts it much, much lower (cry). $300 each on the monitors, $60 for keyclone.

With the money you have in already, I see no reason why you couldn't take the two computers you have and maybe pick up a a simple third one to split them up a bit more. My general theory is 1 monitor per PC and I have had very little problems. I have always had problems trying to run two games on different monitors of the same machine. That being said, I have heard nothing but great things about running SSD and am planing on doing that myself once the holidays are over and I recover. I will be looking at the suggested disabling of the sound cards on my slave machines though.