H Visas: H1B Transfer

Hi,

I am currently on OPT and I got H1 approval and I got my documents too for Company A. My h1 for company A starts on October 2008.

My questions are

1, when can i go for H1 stamping and where is the best place and what's the procedure for h1 stamping

2. I got an offer from other Company B. Whether I am eligible for H1 transfer. If I am eligible, whether I can apply for H1 transfer immediately or Do i need to wait till October.
If not, When can I apply for H1 transfer and what are the things required for H1 transfer.

Thanks a lot for your time and reply.