I am planning to go with 18x8.5 wheels with 255/45ZR18 rubber up front (27" tall) and 18x10's and 295/45ZR18's in the back (28.3" tall). IMO a wider wheel on the back looks way better. The Nitto 555 line has a good selection of 18" tire sizes available if you haven't checked them out already.
I don't know if you have seen this tool but it is pretty cool for checking out how various tire combinations will work together.
http://www.miata.net/garage/tirecalc.html
Picking tire and wheel sizes is a painful process isn't it.