I liked the XtrackCAD free software (for HO), for code 83 libraries using Walthers Shinohara turnouts, Atlas flextrack and crossings, but had three winters to tinker with it while deciding whether to dive in. It met my needs well. Use the tutorial if you try it. There’s a Yahoo user group to get any quirky questions answered.
Check out these recent threads, perhaps useful:
http://cs.trains.com/mrr/f/11/t/214733.aspx
http://cs.trains.com/mrr/f/11/t/214129.aspx
Another issue is how to translate a software plan to actual layout, which the first thread also addresses.
Also, I found many of the layout and track planning booklets and databases on the MR site (directly or to order) an important part of the planning. And, post a plan for folks comments and you will be aware of improvements or “watch-outs” that you may miss on your own.
I found the planning part rather interesting. Good luck.