US firm contracting and/or operating in Belgium

I have a US Limited Liability Company (a partnership) which does business in the US. We are a design and web development firm, providing web application design and development as well as web strategy (e.g. content strategy, IA).

One of my partners recently moved to Belgium (I'm not sure what the exact visa is, but it's a long term resident visa based on a spouse's work visa). Given that she resides in Belgium, what kind of tax and legal liabilities would we face if we contracted with a business or organization based in Belgium? Does it matter if she performs any work on that particular contract?

If we need to create a Belgian entity, is there a large cost difference between forming a subsidiary vs. a branch? Related answers on OnStartups indicate that a subsidiary provides more liability protection. Would love to hear if anyone can point us to any good resources in Brussels too (so far the advice we've gotten from BE has been inconsistent).