of the users at Expreview forums has posted the complete GTX 460 768MB press kit and according to the performance slides, the upcoming GTX 460 768 is a bit faster than the "generic" HD 5830 and a bit slower than the GTX 465.
The slides refer to standard GTX 460, so we are looking at reference cooled card working at 675MHz for the GPU, 1350MHz for shaders, 3600MHz for 768MB of GDDR5 memory paired up with a 192-bit memory interface. This means that the GTX 460 1GB should end up even closer to the GTX 465 card and might end up to be even faster in some cases and we don't even want to mention factory overclocked cards.
The HD 5830 manages to outperfrom the GTX 460 768 only in Unigine Heaven v2.1 benchmark, while in other benchmarks and games, GTX 460 768 is faster. Of course, bear in mind that these slides come from an Nvidia partner and we would surely like to see more benchmarks.
You can find the slides here