Hey Xzin. I multibox 5 shaman on Suramar (level 54 atm). I run all five WoWs on one computer with the specs below. I broadcast keystrokes using an AHK script. I also have a script that logs all five clients in simultaneously. Can post those scripts if anyone is interested.
Specs:
Intel Core 2 Quad 2.4 GHZ
320 MB NVIDIA GeForce 8800 GTS
6 GB RAM
2 x 300GB SATA 7200RPM in a RAID 0 (speeds up read times substantially)

The most important mods I use are Twoboxtoolkit and Automaton.
I tried multiboxing 3 characters on different machines at first, and definitely prefer one machine. There are many tricks to doing it, so if anyone has any questions feel free to ask.

@Gallo: Yes you can put multiple video cards in one machine. Though depending on the cards you're talking about, there could be power concerns. Make sure you've got a hefty power supply for high end cards.