Depending on the size of the client company, and also the country you're in, I'd be cautious about giving a gift, unless it really is a token. Many large companies prohibit employees from receiving personal gifts from suppliers, so it could create an embarrasing situation of the gift is to an individual rather than the company as a whole.
One solution might be to give, say, a box of chocolates that can be shared by the employees within the department or team.
That said, an email or a card thanking the client for their business is always a good idea. And I agree with Shaun that it should be done after you receive the final cheque.