Non-compete clauses and provisions are often unenforceable, depending on the jurisdiction.
For example, in California, as per http://www.google.com/search?q=non-compete+california:
It is considered a fundamental policy of the State of California that agreements in restraint of competition are to that extent void. The California Supreme Court, in Edwards v. Arthur Andersen LLP, 44 Cal.4th 937, unanimously held that Business & Professions Code Section 16600 invalidated a provision in Edwards' employment agreement that restricted him from servicing customers and competing with Arthur Andersen following the termination of his employment. Notwithstanding the general premise that non-competition agreements are invalid, specific Sections of the B&P Code provide certain exceptions to California's policy against enforcing non-competition covenants which apply in limited circumstances.
In the UK, as per http://en.wikipedia.org/wiki/Non-compete_clause#Europe
In the United Kingdom, CNCs are called Restraint of Trade clauses and may be used only if the employer can prove a legitimate business interest to protect in entering the clause into the contract. Mere competition will not amount to a legitimate business interest.
Moreover, if they've terminated your employment at will, and the field at stake is your only area of expertise, it would also not be reasonable of them to expect that you have to earn no salary (or equivalent) for 6 months (or until you learn a new trade in order to produce comparable income). If they do pay you a special severance package, then that's a different story.
Likewise, if you did a clean room design of your web-site, it would certainly be rather preposterous of them to claim that they own your new web-site as well. (However, if they had some patented know-how, then that's how they could get you.)
Nonetheless, to the contrary of the above, based on the information given in the question, I'm not sure I understand what would be the fundamental difference between an affiliate site that compares cars versus one that compares alloy wheels, they potentially sound to be quite related to be differentiated as separate fields of business.