The best advice I can give you is to perform a site survey. Once you know how bad your signal propagation issue is you can design your network to deal with it. You'll need a laptop, so if you don't have one see if you can borrow a friends.
First, you will need some wireless stumbler software, preferable one that retains the history and provides you with a signal strength vs signal quality graph. This one looks pretty good:
http://www.chimoosoft.com/products/apgrapher/
Second, place your access point/router near the center of your home as this will give you the most coverage in a single access point design.
Next walk around the house with the stumbler software running and note any points where the signal is weak (especially if it cuts out completely). You may want to walk around outside your house too, to ensure you aren't unnecessarily covering your neighbors house instead of your's.
If the signal strength and quality were good throughout most of the house, you may be able to move the AP/router to better cover the weak spots. However, if you have several places where the signal is weak or non-existent then you will need to get additional wireless boxes.
Some boxes can wireless extend the network, others will require a ethernet connection between the two boxes. Just be sure to use the same network name for all boxes and only grant one the ability to distribute IP addresses.
If you do need to add access points, I'd suggest you add one at a time. Place the boxes where you think they will give the best signal and then repeat the site survey. If you get the kind that need to be inter-connected via ethernet, know that you can complete the site survey without connecting them to each - this make it easier to move them.
