The miracle of how much stuff you can push through a single coax is right up there with the great pyramids.
The current limit on RG6 is roughly 1.5Gbps (single-link HD-SDI). Extron HD-SDI-certified RG6 (18 AWG) can do this up to around 285 feet (often longer in practice, depending on conditions).
I have a 500' spool of it in my office right now...
EDIT: My bad. I forgot about SMPTE 424M (3G-SDI) because that is typically run over RG11. It doesn't have to be, though. 3G-SDI can do 3Gbps over a single-link of RG6.
Clark Electronics' Low-Loss RG6 Coax claims they can do up to 376' at these data rates, though I bet in practice you'd see much, much less usable distance.
For comparison, they claim their RG11 cable can do up to 570' at 3Gbps, and 849' at 1.5Gbps (which would be more typical for a home HDTV cable feed requirements).
If you're going to run something yourself, and bury it, I'd buy RG11. The main problem with RG11 is that it is SUPER non-flexible, so it is not appropriate for in-wall use, unless you can do everything in a straight-run.