As I understand it, strictly a "taxe" is an arbitrary levy that the government decides to take on something (e.g. they might decide to tax 20% on cigarettes, but there's no direct correlation between that 20% and the "cost" of cigarettes to the government), whereas an "impôt" is in some way directly related to the cost of running particular services.

But there is a little bit of ambiguity in how the two terms are used, and in practice they tend to boil down to what I think in English would just be called direct/indirect taxation. See:

http://fr.wikipedia.org/wiki/Taxe


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Note added at 12 hrs (2010-04-26 22:29:16 GMT)
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Another view from a textbook I have in front of me Cloose, E. (2004) "Le français du monde du travail", aimed at students taking vocational business qulifications: "Lorsqu'il vous touche directement, c'est un impôt, par exemple l'impôt sur les revenus, lorsqu'il vous touche indirectement, c'est-à-dire qu'il s'applique sur quelque chose que vous achetez, c'est une taxe [...] Impôts, taxes, deux mots différents pour désigner peu ou prou la même chose : il faut payer!" (p. 42)