You make up for the smaller rims by buying tires with taller sidewalls. The overall diameter of your wheel tire combination....no matter if it is a 16, 17, or 18" wheel, should always be around 25.6" inches. Popular compatible tire sizes are 245/50/16, 275/40/17 and 315/35/17. While all those sizes are completely different, they all have the same overall diameter. 18" wheels need lower profile tires to be in the 25.6" range....what size is on your current wheels?
Either way, if you do it right and keep the overall diameter the same, your car won't get lowered at all, you'll just see less wheel and more sidewall. More sidewall means a bit cushier ride, better traction, but you give up a bit in cornering. To me, 17" provide the best mix of everything....18" is about as big as I would go....and I'd only switch to 18's for looks.
Manny