AOA!

I have been reading on Alloy rims lately,
I am pretty much interested in know thing pro's and con's of changing rims, I have a santro exec, and would need some suggestions too as should i be changing the rims, i am more concerned on the performance issue rather than looks.
I don't want that after changing rims my fuel consumption rises. I would want it to get lesser! Haha...

Does changing rims improve performance?
Also how would i make out that the guy is selling me good rims? how can i make out the good ones?
actually the santro exec comes with imported rims(atleast thats what the say) so i am thinking that are they worth changing to alloy or are these good enough?