Non-compete across multiple state lines?

I signed a non-compete agreement upon joining Company A in late 2001. Company A's HQ are in TX, though I signed the non-compete in CA upon joining a division of Company A that was based in CA.

I left Company A in February 2005, after being transferred to a GA-based division. I accepted a job with a division in Company B, located in CA.

I am leaving Company B to go work for Company C, based in NV. Both Company B and Company C are competitors of Company A, though my joining Company B was deemed as "not a significant threat" and not in violation of my non-compete, as I was going to a non-competitive division of Company B.

Company C is a direct competitor of a division of Company A that I last worked for 14 months ago, though I left Company A only 6 months ago (I worked for a different, non-competitive division in the interim).

The terms of my non-compete signed with Company A state that for a period of 2 years after I have left Company A, I must notify Company A of any intent to accept a job offer with a competitor, and that Company A has the right to prohibit me from working for the competitor (but will compensate me at the same rate of salary/benefits that I was earning at Company A upon my departure, if this prohibition will inhibit my ability to find a job).

My question is - what is the enforcability of the non-compete of a TX-based company across other state lines? I signed the non-compete in CA, and am now a resident of CA again. My intent is to move to NV and accept the job offer with Company C. Is the non-compete enforceable - and do I really have to tell Company A my job plans for the remainder of the term of the 2-year period (18 more months?)