205/40-17's are a little on the small side. It depends on the springs but you can probably drop your car up to 2" with 205/40, 1.8" with 205/45 or 215/40, or 1.6" with 215/45's. Or somewhere around there. Just make sure you're using wheels with a 40-45mm offset. And I like the look of 215/45's myself just because they fill out the wheel well better -- not to mention they ride better.
Oh, and do you have a hatchback or a sedan? The hatchbacks tend to have more problems with rubbing in the rear.
i've got a gls sedan. i guess i'm not looking to maximally drop the car, i just want something tasteful and know what my limits are. can anyone point out some posts of cars (or post yourself) an elantra dropped (not chopped) and just list how much it's lowered. that would be really helpful
thanks all, very helpful i'm looking into the hydraulic coilovers, probably expensive, but for me worth it because of the varying terrain i drive in daily, would have clearance issues getting up my driveway if i lowered the car permanently