Dedicated Server Migration Worries/Stress/Help!? - Syncing Xferred Accts/Data Loss?

We are about to migrate from one dedicated server provider to another... unfortunately I am the one "most knowledgeable" and in charge of this. The new provider, LiquidWeb has proven top notch thus far.

There is only so much they can help with regarding the migration for liability reasons.

I am a bit worried about this migration because we have some business crucial, highly mysql db driven sites.

LiquidWeb will help me with DNS stuff and walk me through things as needed... I just fear I'm not asking all the right questions. Between LW and http://forums.cpanel.net/showthread.php?t=18294&highlight=synchronize+transfer I think it shouldn't go too badly... but the question i really have is data loss / syncing from old server to new. For some of our larger sites (~3-5gb)... I would really like to, and it seems it would make sense to an initial transfer then to a data sync of some sort... but it doesnt seem WHM offers this... what am i missing (related to discrepancies in data / email etc etc between old and new).

How can we test crucial sites after a transfer? IP testing won't work because the sites are dependant on subdomains (bad design i know... we are rearchitecting away from that though).

I was planning on transferring the more static sites on Friday night / Sat early morning, and then transfer the more intense and business crucial / client sites late Sat / Sun early morning... then doing the nameserver change/update.

Last time we migrated (last year)... the biggest problems we had were related to lost web mail. Unfortunately I did not keep a log of the experience, and don't remember too many specifics about it (dumb, I know).

What questions might I not be asking hosts / developers related to DB driven sites? What issues might I be overlooking? I know I'm not the first person in the world to do this stuff... but what do I need to be aware of? What, if anything, is lost in the transfer? I know we will need to replace our customized php.ini and related files... what else? What are some of your transfer / migration experiences?

I know I'm asking bad questions... unfortunately I am handling this, and am going to have to make the best out of it.

Sorry for the noob questions, I know the generalities and lack of knowledge can be annoying, but I really I appreciate everyone's help.