I used to work for a guy named Ryan Toenies at a company named Inetium (now Avtex). Ryan now is the VP of Sales for the Sonoma Partners CRM practice and his company has posted some great information about what to take into account when considering an upgrade from CRM 4.0 to CRM 2011.
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There is a lot of wisdom presented here and I think it gives a good roadmap for some things that need to be considered when upgrading. Obviously it is not exhaustive and you need to consider the implentation guide to really get official upgrade information, but I think this if very helpful and has presented a few things to consider that I haven't thought of before.
Thanks, and I hope this helps!