Absolutely, replace the main shutoff valve while the meter is turned off, as suggested. Get the washerless kind, 1/4 turn brass ball valve w/long handle.

When you're ready to install your angle stop, you have options: compression valve, like the one Houston mentioned, sweat valve and threaded valve, for which you need to install a male adoptor on the new nipple. Don't forget to insert a copper size flange first.