ok, so micros would have to know about every other one too, that would be annoying.
Maybe if you add (again just for 20mhz), 96 to TCNT0 in every overflow, then the effective rollover would be 160 (instead of 256). Does that help?
You should get an interrupt every 512 microseconds at 20mhz that way, as opposed to the usual interrupt every 1024 microseconds.
Of course there be other possibilities too
Change the prescalar to 128 and you are back to 1024.
edit: oops, no 128 prescalar, just 8, 64, 256, 1024 (and 0).