In agreeance/agreement to Thumps statement above in the first paragraph, I wanted to post some aids/tools in calculating correct tire/rim combinations. You do have to know what the stock values are but you most likely already do or you will
This from 1010 tires.com to help in guiding wheel buyers...
Wheel offset calculator
And here's a tire size calculator from the same site...It helps in determining whether your speedo will be off or not and if it is, it'll help determine how much faster/slower your wheel will turn in referrence to the stock size wheels.
Tire Size Calculator
This is just a guide and not the bible to buying wheels.
Hope this helps...