I think it really depends a lot on how big you were to start with, and your individual body type. I started off at 19st, am now 13st 10, and people started to notice after I'd lost about four stone. Under all the blubber I have a very tiny frame though, so I think I'll have to lose another couple of stone or so before you can see a major difference.
Dress size is also an individual thing - I've gone from size 26 to size 16/18, which is actually better than I was expecting considering I'm only halfway to target.