Re: Leaving a job/contacting clients
I've looked at all five previous answers, which give conflicting views, and I'll give you my opinion and then a couple of references.
Mr. Guerrini - accurate
Mr. Brainard - wrong
Mr. Nelson - wrong
Mr. Gee - a bit rambling, but accurate
Mr. Fernandez - raises a side issue.
I suggest you look at a couple of statutes. The first is Business & Professions Code 16600 to 16602. 16600 declares that contracts restraining anyone from engaging in a lawful business, trade or profession are, to the extent of the restraint, void; 16601 and 16602 go on to create two important exceptions which are almost certainly inapplicable to you -- sale of the goodwill of a business, and disassociation of a partner or major shareholder -- where restraints on competition MAY be permissible.
The second relevant statute is the Uniform Trade Secrets Act (UTSA), found at Civil Code sections 3426 to 3426.11. The UTSA is a pretty broad-scope law, covering trade secrets of all types, but of concern to you is that some things you might "pick up" - figuratively or literally - from a former employer aren't yours to take and use in your new, competing, occupation. This includes things "removed" from sensitive files, the boss's Rolodex, etc., and probably most stuff you were told, or should have known, was sensitive, confidential material. Read the definition of a trade secret in the Code, and avoid using anything that might meet the definition.
To be more specific, courts have generally ruled that an ex-salesman can notify his former clients that he has left, and is either working for himself or for a competitor. Further, it is generally permissible to accpet their business if they decide to follow you. Where things get dicey is when you begin to utilize customer-specific information you developed while at the old job, such as the Purchasing Manager's favorite Scotch, the fact that they always buy on Tuesday but throw out salesmen who show up on Friday, and that kind of thing -- these can be trade secrets of the former employer.
Finally, I should point out that this whole area is often the subject of threatening letters and lawsuits without much chance of winning, but calculated to bring about a settlement in favor of the former employer. So, maybe the question you need to ask yourself for strategic purposes is, "Do I want to risk being sued and having to defend, even if I'm 99% likely to win?" On that point, I have no advice to offer, but I'd guess that resisting intimidation usually pays off.