First, let me congratulate your choice in (possible) schools! UW-Madison is amazing in so many facets---academics, social opportunities and athletics. Currently, it has produced the most Peace Corp volunteers, as well as the most Fortune 500 CEOs---which illustrates the diversity of student interests and strengths. I am so proud to have graduated from Wisconsin, and have yet to encounter an alumnus that feels otherwise.
The Greek system at Madison is strong, but not overwhelming. Many people join, and many people don't. It's one of the largest schools in the country, and there really is a niche for everyone. I joined as a sophomore, and it was one of the best decisions that I've made. I love my sorority dearly, and made my closest friends in the house. I didn't find recruitment all that cut-throat, and was lucky enough to have to cut houses each round (rather than having them cut me). And, in the end, I got my top choice. I graduated 8 years ago, but have heard that the system is pretty much the same. If that is indeed the case, the current sororites all have their unique strengths, and you should have no trouble finding one that is right for you. Good luck! And, if you have any specific questions, feel free to PM me.