I should be getting a GF8800 upgrade card for my 1st generation MacPro next week. I'm getting it as I bought the GF7300GT originally, expecting to be able to get a better card later, and not wanting to pay significantly more (and wait longer) for the X1900. So I will be able to give you some info, but I will not be able to give you a direct comparison against the X1900.
From the reviews I've read, I believe the GF8800 is not a significant improvement on the X1900 for normal Pro applications, but that it should be better for games.