For the phones I would make sure your telephony switch supports VoIP handsets and wire anywhere you expect to have a telephone with PoE. All the VoIP phones I've used have a built in switch so you can plug the user's workstation into the phone. Newer phones will have a gigabit switch. I've done testing with Avaya and Cisco phones and I couldn't find any bandwidth limits when plugged into the phone vs. straight into the wall jack. It's a great setup because you don't need power dongles or redundant wiring for the phones. Each desk has one jack for voice and data.
Check the power draw from the phones and make sure your switch can provide enough. I've seen PoE switches that can't light up every port because some devices were pulling too much power. Current-generation phones seem to take far less power than they did a couple of years ago unless they have big fancy LCD screens.
This may require an upgrade to your PBX though, which can be expensive. For the size organization you're talking about (or for just about any size, depending on specific features you need) going with Asterisk might be a great idea. You get free conference bridge support, voice mail, menuing, etc. and can perhaps ditch the service contract you're paying right now. You can find IP phone service as well or if you want to stick with T1s (or copper if it's cheaper), Digium sells hardware to support that which is generally pretty affordable.
And, unrelated to the phones, you might want to put LAN drops in the ceiling throughout the office for wireless access points.