I have seen many Optimas with 18" wheels. I think that they look a tad gaudy. 17" look nice, but 18" is just pushing it on a stock Optima. If your Optima gets lowered some, 18" will look fine. But, with stock ride-height, they look out of place.
Just my .02, and I'm not as "gangsta" as many other people on here.
- Tum
P.S. Also, re-consider getting a CAI. A nice SRI is just as good (unless you are stuck in traffic) and you don't have to worry about puddles, rain, etc messing up your engine.
Once again... just my .02