CUDA-based software can detect my GPUs

I just built a new Windows 7 (64-bit) system with:

  1. GeForce 6150SEnForce 430 on the motherboard (which I use for the monitor), and a
  2. GeForce 9800 GX2 (for computing)

I have yet to get any CUDA-based software running to demonstrate to me the computational speed-up possibilities.
For example, I tried running BOINC and FastFractal256, but neither detected any GPUs.

So, I thought I should update my driver – I already have 185.93 installed.
Every time I try to install the latest or newer driver for the 9800 GX2, the system bluescreens on restart!
I think it’s doing this because the driver installer is updating the driver not just for the 9800 GX2 but also
for the 6 series display driver on the motherboard. The driver is supposed to support both GPUs.

And then, after I do get the driver working, do I then have to install a CUDA driver?