[Erik de Castro Lopo]
The problems you get when doing a discrete time
simulation of non-linear
processes are mitigated as the sample rate is increased. Therefore, the
higher the sample rate the better.
For guitar distortion, I've found that at 4x oversampling the quality
starts to sound bearable, and 8x is what the CAPS units use (with an
up- and downsampler FIR filter of 64 taps each).
When I started working on virtual amps, this page helped a lot:
http://web.archive.org/web/20051128104636/http://www.eecs.umich.edu/~mmccor
q/tubes/simulation/simulation.html