One report indicates that for the NV+10.6.4 driver combination, that disabling the "multi threaded GL driver" can smooth out frame rate without too much of a performance hit.
-glmdisablemtgl <- is the launch option for that, it has to be in the launch options Steam panel, not entered by hand in the console.
Another option you have that could help, is to shut off the engine's use of "occlusion query" technique. This can have some measurable performance wins but it is really a last resort because disabling it can potentially mess up the engine's ability to correctly calculate dynamic lighting level - i.e. you could wind up with scenes that are too dark or too bright. This one is a console variable and can be toggled live. One of the highest priority tasks we have is to make it so this feature runs correctly without a performance cost, and that work is ongoing.
+gl_nullqueries 1 <- launch option syntax with a leading '+'.
Setting gl_bufmode to 1 can help and that's going to be on by default in TF2 / HL2 / Portal on their next updates. It is already 1 by default on CS:S. This is also a console variable and can be toggled live, or put permanently into launch options dialog using the + syntax as follows.
+gl_bufmode 1 <- launch option syntax with a leading '+'.
