I have seen many posts and forums on recommended hardware to run Untangle on as a security UTM device. There is actually a really nice rack box that we worked with a client on a purchase to get Untangle up and running and it is from a company called ABMX Servers. They have a great 1U box that is actually configured specifically for network appliance duty.
We spec’ed out a box with 8 gigs of memory, 500 gig SATA drive, DVD-ROM for loading, 2.4 GHz Xeon, and 4 network ports. Untangle x64 7.4 was loaded along with a Premium Untangle package and the box is screaming. Great value for the money and it wasn’t a huge investment for the client.