Thanks!
But i assume that 215/45/17 tyre will be heavier then the 205/50/16, so i'll save some weight from the new alloy but will lose some with the new tyre.
Not necessarily...
Firstly, you need 215/40, not 215/45
Secondly, the 215/40 tyre might actually weigh less than a 205/50 - don't forget there's a 17" hole in the middle, instead of 16". However, it's 10mm wider, so that will counteract the shallower sidewall. Any difference in tyre weight is most likely to be down to construction, rather than size (at the two sizes we're talking about)
You can certainly get 17" wheels that are lighter than the standard 16" items - I know, I have some fitted (7j x 17" Team Dynamics ProRace 1.2 - just 7.9kg)
For reference, the stock wheels weights:-
20vt and LE - 9.1kg
20vt Plus - 9.3kg
20v VIS - 9.3kg
16vt, 16v n/a and 20v n/a 9.0kg