Depends on your budget I guess. There exist quite some (free) plug-ins out there which let you create all sorts of 8bit instruments you can use in softwares like Fruity Loops or Reaper (both not free but as far as I know there exists no really decent freeware for arranging your instruments etc.). If you consider spending a little bit more there also exist many options - I often heard good things about Reason when it's about 8bit music...

BTW doesn't a topic about 8bit music already exist here? I still remember writing some links for said plug-in-instruments not too long ago...

EDIT:
Found it:
http://www.opserver.de/ubb7/ubbthreads.php?ubb=showflat&Number=357845&page=1