I used zentyal for almost a year using 1.5 - 2.0 .. but just try 2.2.1 on Oct 24th 2011, i use Dual Core Atom D510 1,66Ghz, memory 3gb, casing CFI 150watt, almost DVD size, fanless trough out this whole time since 1.5.
feature setup for gateway, FW, proxy+dansguardian+no-ads, trafficshaping, DNS, DHCP, antivirus, PDC, fileserver, monitoring, and GUI always on.
daily traffic average almost 1gb, got user with movie fanatics, kid's playing online gamers.
i remember usually system load under 1, never goes higher than 2.
this is what i got from monitoring after change into 2.2.1 for a view days. but for memory usage .. its always high ..