This can happen if you do a dist-upgrade these days. First GLX is working, after the dist upgrade it doesn't work anymore. That is awkward and besides who can do without wobbly windows and 6-desktops-on-a-cube?
The problem is that Squeeze has recently declared a few older nvidia cards as legacy. However, the detection that a particular card is legacy doesn't work yet. Secondly (but I am not 100% sure about that) it seems that the nvidia-glx driver is dependent on the new nvidia-kernel, and that kernel is not available in Testing, you have to get that from Unstable. But that is a known bug. (http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=549869
There is a good manual here: http://wiki.debian.org/NvidiaGraphicsDrivers#Libraries
, and the bug report points to this page. However, it doesn't work because the detection algorithm does not detect that some older cards are legacy and should not use the newest driver.
First you should visit the nvidia.com web site, specify your card and find out what the latest usable driver version is. When it is 185.xxx you can simply use the newest driver from unstable. If you must use a legacy driver, there are options to solve this, assuming your version is 173xx:
1. Use this page: http://wiki.debian.org/NvidiaGraphicsDrivers#Libraries
and perform steps 1, 2, 3 as specified, but you have to issue this:
2. Do it according to the bug report:
apt-get install nvidia-kernel-legacy-173xx-source
apt-get install module-assistant
module-assistant ai nvidia-kernel-legacy-173xx
Again, you might have to change to Unstable in your sources.list to get the correct versions.