1. The FCE is part of -- let's say -- a "series" of graded tests (KET, PET, FCE, CAE, CPE).  It should be taken by candidates whose level in English is approximately upper-intermediate (in Europe, level B2 of the Common European Framework of Reference). The TOEFL has no such "grades", so a candidate can sit for the exam irrespective of his/her level.

2. The FCE has a "pass mark" (around 60-65% of the total marks), while in the TOEFL a minimum pass mark doesn't exist. This means that weak candidates are not failed; they simply get low marks.

3. One can pass the FCE with A (approx. 80-100% of the total marks), B (approx. 75-79%) or C (approx. 60-74%). Those who take the TOEFL receive a score ranging from 0 to 120 (in the Internet based format, IbT).

4. The FCE consists of five papers: Reading, Writing, Use of English, Listening, and Speaking. The TOEFL IbT hasn't got a section on grammar, so it consists of four parts: Reading, Writing, Listening, and Speaking.

5. FCE candidates have to write their answers on booklets provided on the day of the exam (for papers 1 to 4, that is, for each part of the test except Speaking). TOEFL takers (IbT version) sit in front of a computer and type their answers.

6. FCE (as well as KET, PET, CAE and CPE) tests general English, while the TOEFL focuses on academic English.

As you've already taken the FCE, I will now go on to some specific points about the TOEFL IbT.

7. READING: TOEFL readings are (guess what!) academic and much longer than those appearing in the FCE; the format of the tests is different, too. You have generally to chose from predefined answers (sort of FCE "selective cloze") and might be asked for the meaning of a specific word, for synonyms etc.

8. LISTENING: During the TOEFL IbT listening part, you are made to listen to two types of listenings: 1) excerpts from lectures whose subject can be pretty much anything, from archaeology to biology, from sociology to astronomy; 2) dialogues related to university life in general (e.g. a student asking his teacher an extension; a student borrowing a book from the library; two students talking about the modules they wish to take etc.)

9.  WRITING: For the TOEFL IbT writing part, you have to type on the computer two essays (allotted time: 30' for each composition). One of them will be an academic essay (no prior knowledge of the topic required! They will make you listen to a piece of a lecture and read a test, usually dealing with the same topic as the lecture but with a different perpective, and then you could be asked to compare and contrast, or to summarize, contents coming from both the reading and the listening).  The other will be an ordinary essay (persuasive, argumentative, balanced etc) where you might be asked to give your opinion (no opinions in the academic essay, though!).

10. SPEAKING: This is daunting!!! In Cambridge exams, speaking tests are (usually) taken in pairs and in front of two examiners. Forget about that in the TOEFL ... you have to talk to a computer, in a microphone!  Some of the questions have to do with university/campus life situations and some with academic lectures (for instance, you might be asked to summarize the main points of a lecture).  In Cambridge exams, you answer a question on the spot. In the TOEFL, you have some time (not much!) to prepare your answer.

10. Cambridge tests are "for life". The TOEFL expires after two years.

A small comment, now. Whether the FCE is enough depends on what you're going to do with the certificate and where.

That's all that came to my mind. If you've got any specific questions, feel free to ask.