I think a good guideline for software boxing is each WoW should ideally have 1 core and 0.75GB of RAM. You can get by with less, but you will suffer on framerates. For graphics, maybe we could have a list of cards that should be able to handle 2, 3, 4, etc clients. For example:

4 clients-
Nvidia 8800 Ultra
Nvidia 8800 GTX
Nvidia 8800 GTS 512 (G92)
ATi 3870 X2

3 clients-
Nvidia 9600 GT
Nvidia 8800 GT
ATi 3870
ATi 3850

2 clients-
Nvidia 8600GT

etc...