Here are a few links that might help, but I can't vouch for most of the rankings. In fact some of the chains that are highly rated are not near the best BBQ IMO, but you can read the reviews and get a better idea of which ones you might like.
http://citysbest.aol.com/san-antonio/best-dining/barbecue/
http://citysbest.aol.com/houston/best-dining/barbecue/
http://citysbest.aol.com/austin/best-dining/barbecue/
http://citysbest.aol.com/dallas-fort-worth/best-dining/barbecue/
Personally I think the chain of Rudy's BBQ, and Rudy's Country Store & BBQ restaraunts are waaay overrated, even though they have a good following. Don't get me wrong, it's ok, but it's just not great, and it certainly doesn't live up to the hype. Bill Miller's BBQ is probably the biggest BBQ chain in Texas. Again, it's decent, but nothing to write home about either, and there are better places unless you just happen to be in a rush and need to use the drive through service. County Line is a chain of "upscale" BBQ restaraunts, and it's the same thing there. Pretty decent, but not worth it IMO when there are better places out there.
As a rule of thumb, you will not find the best BBQ at the chain restaraunts. Also as a rule of thumb, the nicer a place looks, the less you will be impressed by the BBQ. I've had the best luck in Mom and Pop places that look like they haven't been decorated, updated, or repaired in at least 20 years.