Reader Arun Sandeep asks: "I want to setup a wireless LAN in an area of about 500 square meters. I will need to be able to connect to about 200 computers (desktop/laptop). I would like the network to have connection so that all the computers can get connect to the Internet. To get this done, how many access points should i have? Is it possible to share folders/drives on the network? Will I be able to connect to the local server and access a local database on the server?"
What you are asking is possible. I will give you a basic overview of what you will need to think about doing. Even if you had sent me detailed drawings of your building, I wouldn't be able to give you an exact number of access points due to several potential constraints, including: How much metal is in the building, how the building supports, such as columns, are arranged, how much open space is in the building not taken up by walls, etc. Let me put this in perspective: You could put a given number of access points in the building and have excellent coverage. As you add people, furniture and fixtures, you would probably start to notice a decline in coverage and would start finding "dead" spots in the network. Adding additional access points or changing antennas to increase coverage will then be needed to get the coverage back to uniform quality.
I would recommend using commercial access points such as those from Cisco or Intermec. You will find the commercial access points to be more flexible -- you can use different types of antennas to maximize your coverage without providing enough of a signal for people outside your building to latch onto. Use the highest level of encryption, such as WPA2, and consider adding an additional layer of authentication, via a RADIUS server, for example.
You will need to connect the wireless access points to switches that are connected to each other so that you can see the servers and database systems that you mention -- and to allow file and network sharing. You will want to think about putting together a network of switches so that the wireless access points can connect to a "backbone" so that all the resources you want to share will be available to all concerned. With the number of computers that you mentioned, you will want to think about a gigabit-speed connection between the switches so that two or three workstations don't consume all the network connection and prevent others from getting the access they need.
As to the Internet connection, this is possible by adding a firewall between your network and your ISP. There are several good candidates. As with just about any choice, you can also look at using open-source options. The thing to remember is how much time to you have to keep things running and your ability to support your self as with most cases of open-source software, the commercial support options can be very limited to non existent.
Depending on your amount of network experience, you can handle this project on your own. Even if you do feel comfortable with doing this yourself, though, having someone else look over your design can help save some headaches down the road. What you start with today is going to evolve over time as your needs change and your company grows.