Limit of sequence

Hello all,
I use sequence to generate an ID for each row by using a same sequence for
all tables (it's a same as an OID). I want know the limit of number
genereted by a sequence. Did sequence use short or long integer?
I use my own ID instead of system OID because I can't use it as foreign key.
I use now a 7.1.3 version of Postgres. Is the 7.2 version ready or not yet
and did it support the reference to an OID.
Thanks.
Mourad.