Just a follow-up comment:
This is definitely gnome-related and not either X or ppracer.
Using the fluxbox window manager, running ppracer does not freeze the
system. Attempting to run ppracer in gnome freezes the system totally, and
even attempting to kill X from an ssh login to the machine fails - only a
hard reset will stop the madness, and ctrl-alt-F1, F2 etc do not respond
when this happens in Gnome
To conclude: the problem appears to be in Gnome's detection of refresh
rates, or something of the kind.
Since ppracer is , shall we say, popular... ;-) , and an Nvidia FX 5500 is
hardly unusual, perhaps this bug should be raised in importance, assuming
that it is reproducible.