So far, according to an article on Extreme Tech, ATI is in the lead by a healthy margin. However, some of this is suspect due to an OEM agreement between ATI and Valve Software (makers of Half Life 2), which Valve claims came after the benchmarks were made.
The benchmarks show as much as a 100% performance gain by using the ATI Radeon 9800 Pro vs. the nVidia FX 5900. However, they do include some variations based on the specific configuration parameters and there are some interesting specifics in the graphs and text of the article.
If you're interested in graphics boards and their performance characteristics, it's well worth a read.