6800 GT would be the better choice.
Here's a few specs:
GT: AGP 8x
GL: AGP 4x
AGP is being booted out anyway because of the migration to PCI Express. 8x is still not bottlenecked though, but you want at least 8x.
GT: RAMDAC Clock Speed - Dual 400 MHz
GL: RAMDAC Clock Speed - 350 MHz
Plus the larger texture memory and higher refresh rates. And because the GL is an older card, it may not have support for OpenGL 2.0 or DirectX 9.0. I couldn't find any information on that so not sure.
FireGL is a lot older than the 6800 GT as well. Plus you can overclock the GT and unlock dormant pixel pipelines to make it even better!
The FireGL was rated for use with Maya 5.0. There's no mention of it in the specs on 6, 6.5, or 7. The recommended FireGLs for that are the V series.