I've been fumbling through this problem with my FX5700 Ultra and a Samsung SyncMaster 243T (24" LCD, native 1920x1200). I think I've finally decided that the reduced pixel clock that I see under the latest (7664) nVidia driver is a limitation of my graphics card. Well, not mine exactly, mine will happily run quite a bit faster.
It seems like nVidia may have changed the latest drivers to enforce the expected/supported TMDS clock rates for their cards. (Aside: don't confuse the TMDS (DVI) pixel clock limit with the RAMDAC limit (like I did at first)...they are separate chips/circuitry and probably have different specs!) Since alot of nVidia chips can actually go faster than the specs that they are sold under, people can get things to work that, according to the card's supported capabilities, should not.
The work around that I chose was to go back one rev on the driver. For me the reduced pixel clock modes wouldn't work as my monitor really, really wanted a very specific 154MHz pixel clock 1920x1200 format. The new driver wanted to limit my FX5700 Ultra to 150MHz.
I bet the limited pixel clock that people are seeing is a function of your card. Do ppl want to post their card model along with their pixel clock limit under 7664? I bet that might confirm the pattern. If this fact is true, we may not see this "feature" disappear in future drivers. It sure would be nice if nVidia added a flag to disable this limitation.
You can overclock lots of other parts of the card with things like nvTweak...
In general, I do like the strategy of examining your monitor's EDID output (or manual, I guess) to find the exact native format and then setting that up in your xorg.conf. If you have to, tune it a little to reduce the pixel clock. Just remember, sometimes persnickety monitors might not like it much when you deviate significantly (or even minorly!) from the timing format that they expect. My Samsung appears to be one such monitor. In fact, mine is so picky that even using the EDID reported modeline only works if I also specify "ExactModeTimingsDVI" in the configuration file. I wish I knew what the heck the driver was doing with that flag...then I could probably get things working under windows too!