Linux, PC-based, PCI or USB oscilloscopes?

~
 My son is quite a bit enthusiastic about computer electronics and he
wants to use an oscilloscope for his learning/tinkering around
~
 Oscilloscopes are not really mass-market toys, but very expensive
pieces of equipment, so, I was telling him to check his options and he
would now settle for a PC-based oscilloscope
~
 I am more of a software guy, so, I don't know much about it, but I
was trying to make him understand that he could certainly not use a PC
with a chip pulsing at a given frequency to gauge another one with a
(n equal? or) higher frequency
~
 Am I basically right? What other aspects should be consider while
getting a PC-based oscilloscope? Which other functionality are you
throwing out the window when you use a PCO?
~
 Thanks
 lbrtchx