Here's another good link to compare tire sizes.
http://www.sgmotorsports.com/tiresize.htm#TireSize
I'm not sure what you are after. If you are wanting a lower profile, it will, in my opinion look weird with 15" wheels. Then it's going to change the total height, speedo, etc. Are you wanting to keep it in the 28 or 29 inch range?
Personally, with 15" rims, I would go with 60 series sidewalls. The above link will help you decide, I'm sure.
As a side note, BF Goodwrich was giving away Gtech's with a set of tires at one time. I would have probably bought a set if I hadn't already bought a Gtech.
Speaking of Gtech. Has anyone seen the new ones they have come out with? Nice.
http://www.gtechpro.com/