How can I do this? Do I need to buy an airport express? Will it boost the signal or simply extend the range of the signal it receives (ie does not affect signal strength). Thanks.
It will definitely extend the range of your network.
I have a 4 storey house and use 3 of them to extend the range across the whole house, plus I use airtunes on all of them and also have 2 printers connected.
Signal is now strong enough for me to get online at the end of my garden which is roughly 75 feet long.