Run windows 8 upgrade assistant.
if you think that windows 8 (core or pro) is worth the upgrade, try to dual boot instead of in-place upgrade.
that way when installation fails or become problematic, you still have a working pre-windows 8 OS.
if you are satisfied with windows 8 and all your previous programs are compatible or working fully under windows 8, then you can clean install or do in-place windows 8 upgrade install.
unfortunately, the 39.99$ windows 8 pro upgrade offer is no longer valid.
it ended on january 31, 2013.
the current upgrade pricing is >100$ for windows 8 core and much more for windows 8 pro upgrade via online microsoft store.
windows 8 pro system builder kit costs ~139$ via retail stores