tilly's right. Splitting a week between Paris and London can be difficult, depending on the type of sightseeing you plan to do. The Louvre in Paris, for example, can easily take up a whole day (or more). Much more history has occurred in a much smaller space than here in America.
in reply to "jeffa Goes To Europe"
That having been said, if you're in Paris, I highly recommend seeing Sainte Chapelle -- it is probably the most beautiful cathedral I have ever seen. And no picture could ever do it justice.
Also, get tours when possible: they greatly heighten the experience.