Excellent. Thanks for the info, Brad. As it just so happens I have the DX-SDK installed and I didn't think to check it at all. :D
Can't tell by your sig, but are you running DX10 on Vista? If so, I think we have confirmation that 8192x8192 is the res limit. My MaxTex numbers are identical for my 8800GTS. Have you seen anything that indicates DX11 might have increased these limits?
Also, it hadn't even occurred to me to think of a combined EF + TH2Go system. I'm going to have to daydream about that now for a little while... ;)
I'm Windows 7 but it looks the same in Vista and XP in DX Caps Viewer. What it is showing is the hardware capability, not the DirectX version capability. You have to check the 5000-series card to know what it's max is. All my screenshot shows is that nVidia GTX260 max out at 8192x8192.
One would assume that since these are values taken from the hardware that it would probably be the max limits for OpenGL too. That is an assumption on my part though.