[quote]Scroll all the way down for a iTX system that will also run OS X on it comfortably. For £350 (or $600) you get a very good spec’d system and could mount it all in the case in you OP and on the back of a monitor.
[/quote]
$600 EXLUDING the case, bluetooth, IR & Wifi or warranty on the unit … :-S, a Mac-mini is almost as good value and you can get a Vesa Mount for them. I not knocking the specs I know the Proc / Ram /HDD is better than a mini, if I was going the OSX route though I’d probably go for the real thing, Hackintoshes are great n’all (and I’ve had 2) but you just never know when the latest OSX update will screw everything up.
Anyhooo’s though …
Idea 1.
Why not mount the entire lot in a half decent flightcase and build yourself a 00budget 
http://www.thomann.de/ie/thon_aktenkoffer.htm
You could vesamount a 18’ screen in the back of the case nicely or get creative and completly strip and remount (what i’d do)
You’d have LOTS of room in the base of the case for the mobo, PSU, fans etc..and maybe even for a half decent soundcard & keyboard mounted on top of your choice.
Working out the numbers it should cost under $700 for a i3 3ghz setup with 4gigs ram, 1tb removable drivebay, Wifi, DDR3, 18.5’ samsung monitor and keyboard with trackpad.
Add about $100 and you could get a nice rugged backlit keyboard 
Add $200 and you could get a programmable Point of sale rugged macro keyboard and use it as a controller as well.
Idea 2.
Slightly larger case, 2xmotherboards and a KVM switch.
Idea 3
Modify bigger case and mount 2 x 12’ monitors and 2x mobo’s etc in the bottom … y’know .. Ableton on one traktor on the other blagh blagh 
It’d be some blood sweat and tears but a cool project.
I used to spend hours on the mini-itx site you have my head swirling of the possibilites now 