I recently upgraded Debian to Xorg 2.9.4 and installed nvidia-glx from experimental, version 260.19.21. This was somewhat of an uphill battle as the dependencies for the experimental nvidia-glx package are still somewhat broken. I got it to work without forcing the installation of any packages and without modifying the packages.
However, after the upgrade compiz performance has been abysmal. I am using the desktop wall plugin and switching viewports is really slow - takes a few seconds for each switch. In addition to this, every effect that compiz does, such as zoom animations for icons when launching applications, takes seconds. The viewport switching speed changes relative to the amount of windows on that virtual screen - empty screens switch almost at normal speed, single browser windows work almost decently, but just 4 rxvt terminals slows the switches down to a crawl.
My compiz configuration should be pretty basic. Xorg is likewise configured without anything special - the only "custom" configuration is forcing the driver name to be "nvidia". I've fiddled around with the nvidia-settings and compizconfig trying different VSync settings, but none of those helped.
My graphics card is: NVIDIA GPU NVS 3100M (GT218) at PCI:1:0:0 (GPU-0). This is laptop GPU that is from the Geforce GTX 200 series. Graphics card performance should naturally be no problem.
EDIT: In the end, nothing really worked, and I got really annoyed with the state of compiz and its support in Debian. Many nVidia driver revisions have passed and I am using Gnome 3 now, so I am accepting the best answers to this question even though the issue was not resolved.