A quick browse of release views shows the GTX250=9800GTX+ except where the larger 1GB ram helps the GTX250. There is also a 512MB version though, and that should be about the same as the 9800GTX+. The GTX250 is a better card though as I believe it is cooler and quieter than the GTX+.
The 8800GTX consumes more power and I think would trade blows with the 9800/250 depending on the application.
I never worried about mixing and matching GPU/Chipset manufacturers. For the record, I have a 4850 on a nForce 4 SLI and it runs just fine, though I never tried OCing it on my ATI board or NVidia Board.