Travel during H1B Stamping Expiration Date & H1b Visa Expiration Date (i797A)

[lchalas]

My H1B was approved - given on I797A for 3 years until Dec 2012 .

My H1B stamping visa expiration date - given on passport was until Apr 2012.

1. Can I travel between Apr 2012 and Dec 2012?

2. If so, should I go for stamping again & what do I need to apply for?

3. Also, when is the best time for me to apply for H1B extension?

4. If the H1B extension gets approved, should I again go for stamping?

I would request anyone who have knowledge on this to please give advice for above questions.

Any inputs on this are greatly appreciated.

Thank you in advance.