I measured context switch overhead on my machines.
It's about 7us on my PIII 600 (Celeron) (Coppermine), memory bandwidth of ~ 2.5 G/s, Linux 2.2.18 (SuSE 7.1)
It's about 0.85us on my Dual Athlon 1800+, Linux 2.4.18 (SuSE 7.3)
The code I used to test is test_context_switch.c.