Talking of the HP Microservers this is my set up:
5 N40L's - two with 8GB running FreeNAS for replicated storage, three with 16GB (Yes, it's possible http://n40l.wikia.com/wiki/Memory) running VMWare ESX. All have three NICs and they plug into a managed gig switch.
5 N40L's with the cashback = £500. Lets say £600 including delivery.
RAM - 4 x 4GB = £105.58
RAM - 6 x 8GB = £312
FreeNAS hard drives = 10 x 2TB = £1600 (7.1tb usable per NAS box if you use RAID5).
10 Intel CT Nics = £423.6
Netgear Gigabit managed switch GS748T = £297.58
Total = £3338.76 not including money back from selling the old RAM on ebay.
The article says the total was $7965 which works out to £4901.77 and I'd say that the N40L route is a better route because of the options for different configurations, scenarios, etc.