hmmm... if you are only putting one instance per box then i'd:

o drop the size of the harddrive and spend the savings on a raptor (10,000 rpm drive)... faster loads
o consider jumping to 256M of grfx memory (wow is gpu limited... mostly).
o get a USB2 external DVD drive, cheapest, and save on each box
o 1GB of system memory should be enough... just get the fastest possible for your mainboard
o and make sure they are well ventilated. maybe a top blow hole and definitely good fans

if you were doing more then 1 instance per box, then i'd definitely add more system RAM.

edit:
checked prices on raptors... best was $120. a maxtor 160G SATA drive w/ 8M cache is only $36... raptor is definitely not worth it.