I can't speak for FreeNAS but your current system is more than adequate for XBMC. You really only need a dual core desktop-class processor running around 2.2Ghz (or so) to do 1080p content (reference anemic nettop HTPC's
that squeeze by). Acutally, I would repurpose your E6300 system with 4gb of ram and something in the AMD 6570 or equivalent graphics to XBMC and use the x3 440 and 8GB ram for your FreeNAS system. I'm not disagreeing with StuG's advice, but this route would definetly be cheaper and still perform perfectly well for the same purposes.
Main: i5-3570K, ASRock Z77 Pro4-M, MSI RX480 8G, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod