Personally, I think you may be putting too many eggs in one basket. I would consider getting a second-hand PC and turning it in to a dedicated Linux firewall/security device, and then use your current computer as a media center.

There are three bits of reasoning behind this:
-Media playback/VoIP can be resource intensive, and security apps can be as well.
-If your security box gets compromised, its JUST the security box
-You can harden your security box without affecting the performance of your media box

Just my $0.02