There is a new version of Riva Tuner out there for your tweaking pleasure.
The new version 2.23 offers improvements in driver-level overclocking and fan control for nVidia MultiGPU and SLI sets. Although overclocking on the primary GPU is still tied to all cards in the SLI set, it can be overcome by registry edits.
182.xx driver support finds its way in as does improved hardware monitoring, an improved user interface, improved profiles and a very cool graphing system called stick graphs. With this system you can add any additional graphs you want to the main graph for easier monitoring of multiple items.
However for you Vista x64 fans do not get excited, the driver needed to run Riva 2.23 is not signed yet so it will not install.
Read more here at Fudzilla.
Download Riva Tuner 2.23 here
Driver-level overclocking and fan control on the primary GPU is still mirrored to all secondary GPUs joined into SLI configuration, however this behavior can be disabled via NVAPIMultiGpuClockControl and NVAPIMultiGpuFanControl registry entries if completely independent control is needed. Additionally, the ability to overclock and control fan speed at the driver level for display devices unattached to the Windows desktop (e.g. display adapters dedicated for PhysX acceleration).
In addition, the new version features an improved hardware monitoring module with a new sticky graphs mode specially designed for multi-GPU systems. It is now possible to stick any hardware monitoring graph to other graphs and view multiple overlapping graphs on the master graph. The groups of sticky graphs can be easily edited on-the-fly in both manual and automatic modes, dynamic master graph selection allows you to switch between multiple vertical scaling modes for any sticky graphs group.