If you got approved directly by one of the regional service centers (I guess this must be your case), then I think it will take 2-4 weeks to get your card. During this period, you can't use the approval notice itself to work. On the other hand, if you were approved by local offices after interview, then it can take anywhere between 2 months to one year.
The "stamp" that e17345 mentioned has to be obtained at a local USCIS office. So, if you don't want to wait for the physical card, then just make a InfoPass appointment and go to your local office to have your passport stamped. This stamp is good for both employment and traveling aboard.