Personally, I am not sure if the SEC will go to 16 teams. I think they may stop at 14.
Like everyone said, expansion is all about money. You have to catch a HUGE fish in order for it to be worth expanding. ATM is a big fish. They get the SEC into TX and add a lot of potential TV revenue. Enough revenue that, if they split it 13 ways, everyone gets more. Who else brings that much potential revenue to the table? OU has a big fanbase, but a small state population. I'm not sure if they would be lucrative for the SEC. VaTech at least brings in a new state and the DC market.
I just have a hard time seeing 3 more schools to add to the SEC and they will obviously get first pick of southern teams.
I also don't think the SEC will go to 16 teams unless it is evident that the Big10 and Pac12 are as well. I really don't see 4 more teams that would add enough to the Pac12 footprint to make it worth splitting the money 16 ways. They might end up taking Texas, but I'm not sure who else brings in enough money.
If the other BCS conferences all get to 12, then it will just be one conference with a couple more members similar to the Big10 only having 11 for a long period of time.
If ATM can't leave until next year at the earliest that gives the Big 12 a year to aggressively purue expansion. If we do, then it makes it tougher for other conferences to poach from us. There are a few schools who could add enough to make expansion worth it, but we need to get them now. That would leave the Big East and ACC as more vulnerable then us.