2000 Subaru Impreza What size is it?

Hello, I'm real close to dropping about 800 bucks for a set of tires and rims. But before I do that I want to make sure that the rims will fit.

I have a 2000 Subaru Outback Sport Impreza with a 2.2 liter engine. Currently I have the stock steel rims on there. I want to upgrade to 17 inch rims but I want to make sure that my car can handle it. The reason why I'm bringing this up is because the rims that I want to get are 17 x 7. Someone thought that I currently have 15 x 6.

So what size do I really have? And how do I know what size rim I have? I know that I can upgrade to 17 inches but I didn't know that the rims were so precise. I don't see any markings on the stock steel rims.

So if you can tell me that my rims are 15 x 6 or 15 x 7 that would be great so I can make the right choice. And again. How do I tell what size it is and what is the single digit mean?

Thanks a bunch!
Brian

P.S. I added the URL of the place I want to buy the rims from.

http://www.wheelsnext.com/wheels-rims/Kyowa-KR%5F206-Hyperblack.html