You probably want to start by looking at some architectual data for the building. Depending on the size of each floor and what kind of walls you have as well as whats between each floor. Some materials work better with RF than others. For example dry wall looses very little RF than Metal which reflects RF. After you gather this info, you will most likely want to do some math. An 1100 series AP will give you 6Mbps at its furthest range of 300 ft indoors (open office enviorment). That means take about 100ft off immediatly if you have a few walls up like most building. Than depending on what you have inbetween (concrete? drywall?) you will loose even more. As a start, I recommend reading one of Cisco's Wireless LAN design guide in PDF format at the following link. Just jump to the section "Planning for RF Deployment" on page 37 and read up to about page 58. This is some great info that covers RF as it relates to different architectures, AP Placement, etc. The link is:
http://www.cisco.com/application/pdf/en/us/guest/netsol/ns178/c649/ccmigration_09186a00800d67eb.pdf
