Please open your debug.err file in notepad, and link the contents into a post here. This will give us some technical information that can help troubleshoot your problem.

The file can be found in My Documents\My Games\Elemental

Be sure to copy the debug immediately after the error occurs; if you start the game again, the debug log will be erased and recreated for the new session.

Exactly the same thing just happened to me, on my first turn.  All I did was move my sovereign two spaces, found a city, then I alt-tabbed and the game crashed saying DX: Invalid call (I think that's what it says, will confirm if it happens again).

Debug.err

Windows 7 Home Premium, 64-bit Pentium Dual Core 2.6 GHz, 4GB DDR 3, nVidia GT240 with latest drivers.

Do you want a dxdiag?