Q: Boosting wireless signal

Yes. It doesn't so much boost the signal as allow you to extend the range.

Yes, but if you have a dual band AirPort Extreme, the Express will only extend one band.

If you want to extend both bands, you will need an AirPort Extreme for that service.

Use Airport Utility to configure the AEBS to allow its wireless network to be extended. Configure the AEX to Extend an existing network for which you will need your network's given name and password.

Apple's instructions to configure this are here:

http://support.apple.com/kb/HT4259

Although a second AirPort Extreme is pictured and recommended, the setup for an Express would be exactly the same. The instructions assume that you are using Leopard or Snow Leopard on your Mac, or that have a PC.

If you are using Lion 10.7.3, things are different. Post back for instructions.