it makes me laugh when people say go for 16's cos it handles better and the speedo with be out if you have 17's
i can honestly say that there is no noticable difference at all when you change from 16" to 17" as far as handling goes, ride quality is affected but only very slightly due to the lower profile of the tyre.
after i fitted a set of eibachs i can honestly say the handling was improved and the look of the car too, well worth the change IMHO
as for the speedo being out, have a look below at the rolling circumference below, there is only 6mm in it, if anything it will give you a truer reading
here are the 2 sizes you are looking at fitting-
195/45/16
and
205/40/17
key them in here to see the results-
http://www.1010tires.com/TireSizeCalculator.asp?action=submit
hope this helps you make a more informed decision!