Try DirectX 9 Mode
According to Nvidia's page on the 8XXX series, the Geforce 8800 supports DirectX 10. So I conclude it does not support DirectX 11.
You said that you were using "direct x 10/11 mode". I don't know, but would guess that mode requires DirectX 11 support. (Of course we'd like if it used either DX10 or DX11, whichever your card is capable of, but it sounds like it does not.)
So I suggest simply playing the game in DirectX 9 mode, which we know your card supports.
Its worth mentioning a few other standard things you could try before giving up on DX10/11 mode*, although you may have already tried them:
Both of those are being constantly updated and bugfixed. So checking for DirectX updates is worth it even if a recent version was installed with the game!
*The graphics are shiny after all.