We at Phoronix have just finished up comparing the open-source R300 display drivers against the fglrx drivers -- similar to our R200 article that coincidently came out five months ago to the day. Our comparison can be found @ http://www.phoronix.com/vr.php?view=7481
Feel free to discuss the R300 drivers in this thread. If you are also interested in any other performance metrics or information, just ask.
Some X800 open v. closed results will likely wrap up soon.