As stated, what you are considering shouldn't be a problem. If you go above 16s, you will need to make some modifications to the front which I just recently learned from the previous poster and Techno Duck. Anything 17 or greater, you should change your caster blocks and alignment values. If you want to read about that, search for a post on 968 caster blocks from either ae1969 or Techno Duck. You can read the official Porsche tech bulletin on that post.
BTW, welcome to the forum, it's a great place to read a lot of information about your car and the people here are very helpful. There are a couple more sites that will provide a lot of information about your car as well.
www.clarks-garage.com
http://www.connactivity.com/~kgross/FAQ/944faq05.html
http://www.arnnworx.com/