Most used browsers

Where can I find information on the specific versions of the most popular browsers?

I will be testing a web site that will be going public soon. I want to make sure it looks good on most browser/OS combinations. I’ll start with W98/IE 5.5, but what other combinations are used the most? XP/IE6.0? NT/IE5.5? 2000/Netscape 6.2? Mac9.x/IE5.0?