We have been through this before, to each his own and everything.
With that said, IMHO...20'* look a bit on the large side. I do not mean anything negative towards anyone with 20'* (especially GAMEOVER). From what I have seen though, 17s and 18s to me look very tasteful and nice, without being ghetto. IMHO, those are the perfect sizes for the wheel wells and such.
Not saying it is a bad idea, but just make sure you really really think they will look good in real life before you drop the plastic (credit card).