Yes, a 7.x -> 8.x import will work with little difficulty. If you are importing the whole database, you will want to read up on the parameters of import/export.

Parameters you might want to think about using with export:
COMPRESS=Y
CONSISTENT=Y
STATISTICS=NONE
DIRECT=N
(If you are exp/imp the same version of oracle, DIRECT=Y is _USUALLY_ a good thing. I wouldn't take chances with a version upgrade though)

Parameters you might want to think about using with imp:
COMMIT=Y
BUFFER= (depends on your physical memory)
FEEDBACK=1000