On the networking side of things, don't forget that it's far cheaper to aggregate multiple 1G copper links than it is to implement 10G, but 10G will give you monstrous growth potential. Just remember that it's simpler and possibly cheaper to upgrade those servers with 10G NICs later than trying to deal with a smaller number of wickedly fast servers that are overburdened with their virtual server loads. General-purpose virtual servers won't make much use of 10G for either normal service traffic or disk I/O, but highly transactional applications will, so try to find that balance based on your needs.
Last, remember that server virtualization condenses your infrastructure into fewer physical units, so the better equipped you are to deal with failure of any one of those components, the better off you are overall. And with all the money you'll be saving on power and cooling, adding that second storage array, and firing up replication just might fit into the budget after all -- and that can directly lead to fewer sleepless nights.
This story, "How to buy hardware for virtualization," was originally published at InfoWorld.com. Read more of Paul Venezia's The Deep End blog at InfoWorld.com. For the latest business technology news, follow InfoWorld.com on Twitter.