getvalue() is for retreiving values when the field name is stored as a variable. In your case, you don't need it, since you already know the field name.
Since there is only a single record in the tables, how about:
strlen = gp.SearchCursor(streamtable,"", "","","").next().SUM_LENGTH
artlen = gp.SearchCursor(Arttable,"", "","","").next().SUM_LENGTH
index = artlen / float(strlen)
print index
del strlen, artlen