When you say go voip, do you mean use sip for the stations only or also for the trunks?
My experience (and the experience of those I know) is that SIP trunks don't really work consistently. But, when I say I need to learn VOIP I'm mostly talking about the station side. My goal is to learn enough to build Asterisk boxes to replace key systems. I like the idea of Asterisk because it can use standard trunks for critical lines and SIP trunks for specialized purposes or overflow. (At least that's what I *think* it can do.)
Ah, well, if you want to keep the landlines, then yeah, I guess asterisk is the way to go. If your goal is to replace keyline systems, then asterisk definitely has that kind of support which, it appears, even Cisco's solution does not (from the mouth of Datacraft Asia personnel selling the school Cisco's voip solution).
It can certainly do what you said about using standard trunks for critical lines (extra 'switch' to a plain pots phone on the trunk line in case you lose all power) and sip trunks for specialized purposes or overflow.