回复：Urgent, Xiaobaitu please!

The key is the RD (received date), you can work for new company after RD, the receipt may come a few weeks later in mail. If you are confident about your RD (you won't know for sure before you see the receipt), you can start.

If without attaching your NIW I-140 information, USCIS will,by law, only approve your H1 to Sept 2008. If enclosed your I140 approval and clearly note that in cover letter, you can get up to 3 years of extension. (or two years if that is what your new employer wants)