Nvidia latest cards have three clock speeds that are controlled somewhat by the root clock speed that you see on the slider bars. The Vertex, Shader, and ROP clock rates are controlled by the root clock rate but some have locked-in steps that occur with the root clock. I am not 100% sure about the 7900 and the same exact stepping frequencies but the chip functions in the same way with the steps.
*Notice the chart frequencies at 419, 446,473, 500, 527, and 554 for the root clock rates and corresponding Shader/ Vertex/ROP clock rates.
Check this article to learn more about specifics:
http://www.pureoverclock.com/article32-3.html
If you are hitting a wall on overclocking, bios tweaks although scary at first can help to overclock specific sections instead of all three areas as once. Bios tweaks are not for the inexperienced and a backup legacy PCI video card is recommended for when tweaks go bad.