As much as you can (afford) really ... I used to get away with 1GB XP VM's for most things with a few on 2GB and some simple test ones on 512KB ... but good old MS keeps ramping up the size of things so some of my VM's are 4GB now - so on desktop I have 32GB and my new notebook (that I am just about to purchase) will have 16GB (there are a few 32GB capable notebooks by they blow the budget).

Also don't forget cores ... I use a quad core i7 with hyper threading making it look like 8 CPU's this allows me to have 6 to 8 VM running at a time without much grief.