How rendering routine affect GPU computing perf.?

When kernel is running on GPU, I guess in the mean time, some resource of GPU is used for rendering , i.e. GPU also takes care of what to put onto display .
Assume the rendering process and kernel execution are totally independant. How would rendering affect kernel performance, and how much percent the peak performance (around 400GFLOPS on GeForce8800 GTX) will such rendering process bring down?