thanks a lot for your exhausti've explanation - although it does not solve the issue, at least it informs about the underlying causes and this by itself is valuable.
I will share my situation with you as there is hope also for nVidia users
I have an XFX 790i ultra mobo with a 260(26) GPU and Q6600@3GHz on Vista64. In this configuration all worked very stable including good and stable sound.
Last month I upgraded to a second 260 in 2-way SLI. Immediately and usually when running FarCry2 in top-quality mode the static appeared, usually during complicated visuals (sun lights through trees in jungle or battle scenes). This static would disappear in fps would rise. Moreover when disabling SLI the static would disappear altogether. Please note that other games like GRID and Crysis Warhead did not suffer although they are also heavy from a graphical perspecti've.
The two GPUs are both Zotac 260s with 26 shaders, and one of these cards is a Amp!2 version, which means that it comes overclocked out-of-the-box. The other is standard but can be overclocked manually to the same speed as the Amp!2 (which means next time I will save the money and just do it myself). However, overclocking the standard card to match the Amp!2 would aggrevate the problem. On the contrary bringing the Amp!2 back to stock speed would almost eliminate the problem.
In the meanwhile I further improved my system with 4GB of additional DDR3 memory (@333 FSB) and another WD-Velciraptor in Raid-0. I decided to reinstall everything from scratch and only install latest drivers. I kept the two GPUs at their factory setting (hence the Amp remains overclocked!). End effect is that the system is overall more stable AND that the static in FarCry2 is gone.
Although this is great, I do not understand the reason as more memory or the installation of a Raid array seem remote from the static issue.
If there is anything I can help you with (system log or list of all drivers) please let me know, maybe this helps other users with similar issues.