I played the majority of the Cata expansion with:

i7 920 (stock speed)
8GB DDR3, 1600MHz
GTX 275, 1GB.
Patriot SSD, 64GB



I've upgraded to:

i7 2600K (overclocked on air to 4.5GHz)
8GB DDR3, 1600MHz.
GTX 570, 1GB
Patriot SSD, 64GB (gaming folder)

With an OCZ Vertex 3 as the OS folder.



The processor upgrade makes a huge difference.
8GB of ram is plenty for 5-boxing, but ram is dirt cheap so 16/32GB isn't a bad choice if your motherboard supports it.

The video card was a replacement, because the fan went in the old one...
Did not need to upgrade it at all, for the gameplay.
I run the master on medium-low, the slaves on low.
View distance is maxed on the main, medium on the slaves.
FPS is capped at 60 master, 20 slaves.