Beyond-Tec: Here's what I'm planning on for the PCs. It's total overkill, but I'm planning on keeping these boxes for a while. So, I'm looking for future upgradability. Here's a breakdown of the components -

Mobo: EVGA nForce 680i SLI
Price: $195
Link: http://www.newegg.com/Product/Produc...82E16813188014

Processor: Intel Core 2 Quad Q6600
Price: $300
Link: http://www.newegg.com/Product/Produc...82E16819115017

Memory: Kingston HyperX 2x1GB DDR2 800
Price: $144
Link: http://www.newegg.com/Product/Produc...82E16820134012

Hard Drives: 2 x WD Caviar 500GB SATA 3.0Gb/s (for RAID 1)
Price: 2x$110
Link: http://www.newegg.com/Product/Produc...82E16822136073

Video Card 1 (Main): XFX GeForce 8600GT
Price: $120
Link: http://www.newegg.com/Product/Produc...82E16814150230

Video Card 2 (Touchscreen): EVGA GeForceFX 5500
Price: $50
Link: http://www.newegg.com/Product/Produc...82E16814130255

Sound Card: Creative X-Fi XtremeGamer (or maybe a Fatal1ty)
Price: $81
Link: http://www.newegg.com/Product/Produc...82E16829102006

Case: CoolerMaster Cavalier 3
Price: $60
Link: http://www.newegg.com/Product/Produc...82E16811119074

To answer your question, MultiCasters (I think the correct term is MultiPlexer) work kind of like KVM switches. Only, instead of allowing you to hook multiple computers up to one Monitor, Keyboard, and Mouse, they work in reverse. They allow you to connect one input device to multiple computers and one keypress is broadcast to all of the connected computers simultaneously.

MrLonghair: This is the place I've been looking at to buy the touchscreens from. They're the only ones I can find who have any screens smaller than 12".

CyberTouch: http://www.cybertouch.com/
(10.4" and 15" Orion Series)

The 10.4" Orions and the 15" Orions are $875 each.

Laters!
-The Moo

PS,
After doing some more digging last night, I'm going to take Zin's advice and use something else for the foot pedals. Right now, I'm leaning toward these (CH Products Pro Pedals):

http://www.chproducts.com/retail/pedals.html

Hook a couple of those up and I could jam out like I was playing drums at a Def Leppard show... (Kudos if you get the reference!)