Even with the search engine up and running, it seems the same questions get asked repeatedly in this forum so I thought it would be of some help if I shared three links which have tons of useful info. The first is a very comprehensive wheel spec page which requires membership (free) but definitely is a nice place to visit pre-purchase. The second is a very helpful link for those who aren't familiar with the degree to which various tires and wheel sizes differ from stock. This will help you to figure out what affect your impending aftermarket tire/wheel combo will have on the S2000. The third is a link to Falken Tires but I couldn't continue pointing you to where I originally wanted...that being a size/info file on specific tires and approved rim widths. My advice is to go to a general performance tire such as the FK-451and click on the spec sheet. While all tires have their own specific construction qualities, if you are interested in determining the approximate weight of a particular size tire or the approved rim width variances, it's a fairly accurate place to start. For instance, one look there and immediately you would know a 255 width tire would not fit properly on any 7.5-inch width wheel.
A lot of info for three links and in many cases, a quick visit might get you quick answers.

http://www.wheelspecs.com/main/index.php
http://gs.tolan-hoechst.com/tirecalc.htm
http://www.falkentire.com/html/index.htm