Upgrading to Dual ISPs

I have several competing proposals for upgrading our Internet service to include 2 carriers -- for faster speed and redundancy.  While doing this, we want to segregate the web development group to allocate specific bandwidth for them.  I wanted to ask the community what you think about the situation and how you would proceed.  

We have 2 T-1's and an ASA 5505 firewall going to 2 HP 2510 switches in a Windows 2008 R2 infrastructure.  We have a site-to-site VPN to a 2nd location in another part of the state.

One proposal was to add a WatchGuard XTM-25 and add a 2nd carrier to it.  We actually have that device on loan and working, but they used a 3rd switch and we had to re-cable the web guys to the WatchGuard taking them off the main LAN.  They're "living with" no access to the file server on the main LAN.  We want to get them on a separate subnet because of bandwidth allocation.

The "problem" with the proposals is how to handle email, VPN and bandwidth allocation.  Our switches will handle VLANs but some proposals say VLANs are not necessary.  Our IP addresses will change because of the additional carrier and a change from T-1s to broadband -- we'll save a lot of money.  Some say MX records will have to change, some say they won't.

I'm interested to see what a "community" of people could come up with.

Cheers.