Timing H1B extension with new employer using approved I-140

[girixkumar]

I have I-140 an approved and on 6+years extension that expires in May 2013. I am thinking to change job to and use I-140 to get 3 years H1B extension with the new employer.

FYI I work (and will work) for a big product company, hence EE issues may not arise for me.

The questions I have is:

1. If I switch job immediately (say Mar 1st) and if my new employer uses my current employers I-140 to request for 3 years extension, will the new H1B have validity for 3 years from Mar 1st?
   
2. Or will it still be valid only till May 2013 (current H1B vaildity date) and the new employer needs to re-apply H1B extension again in May 2013? - In this case, I was thinking to wait to get my 3year extension first from my current employer and then switch. Does it matter at all?
   
3. Are there any known complications while obtaining H1B stamping @ consulate with the above situation? Meaning, 3 year H1B extension stamping with a valid I-140 from a previous employer.
   

Thank you very much for any reply!