Neither the XBox or GC will overtake the PS2 in userbase, it has far too much
of a lead and is still selling strong in all three major markets (US/Japan/Europe).
It is possible that either the XBox or GC could overtake the PS2 in software
sales a year or two from now in North America, sort of like how the SNES overtook
the Genesis near the end of the 16Bit era.
However, it is incredibly unlikely that anyone will beat Sony in Japan or Europe.
Currently, 2 PS2s are selling for every 1 GC in Japan. Only 262,000 GCs have
sold in Japan in nearly 3 months... it took about a week to sell 700,000 GCs here.
<< 6 months, a year? >>
Sony has already shipped/sold around 20 million PS2s, and both Nintendo
and Microsoft won't even be able to produce that many consoles until they
are well into their second year.