2 x DC both with DNS, DHCP, GC (global Catalogs), WINS, 1 x Exchange Server 1 x DB server 1 x ISA Server 1 x Security Server 6 would be the ideal suggestion for Servers. Please please please do what you can to at least get the DCs and Exchange on their own boxs, it will save you years of hassle. Remeber it may seem like this is a huge cost to foot at the beginning, but think of the down time it will save, say you only buy 3 server and one goes down, did you just lose your DC and exchange box? how much is that going to cost you in lost data, time and energy to replace and recreate?