Designing a wireless network

from_exp has some good advice and I would like to add a couple of important elements.
In planning the use of a wireless network you must first consider the following things:

• Type of business that the network will be used in. If you are a doctor office, for example, you will need to consider HIPPA compliance. Another example, if your business accepts credit cards you will need to consider PCI compliance. You will need to know what laws and industry standards apply to your business, and how they apply to wireless communication.
• Who is going to use the wireless network and why. Will they access or transmit sensitive data over that network? There are a number of exploits that you will want to guard against, this is where using enterprise equipment can be a huge help.
• What other wireless networks are in the area? If you are in a location where there is a high density of wireless networks there are a number of issues you may experience from channel issues to users accidentally attaching to the wrong network.
• What are the physical attributes of the building? Masonry and metal construction will affect signal strength, and this will dictate where you locate access points.

As for access point density you will really have to consider the physical size of the location, the construction of the building (as I mention earlier) and the technology that you decide to implement. In general, access points also need one or 2 channels of separation.
Even better, as long as you have no adjacent users of the following channels, I would use the non-overlapping channels of 1, 6 and 11. If you have multiple access points you would position them so that you have the channels next to each other (1 next to 6 or 11, and so on) making sure no access points with the same channel are next to each other.
 

Darr247 is right, channel 14 will also work well for you, sorry I did not notice you were in Japan.
802.11a is a great option. Just keep in mind that the typical range is about 60 feet. You will need to plan your network accordingly.

In an office environment you need to be careful of cordless phones.  If you have 2.4 GHz phones, you might go with an 802.11a 5GHz solution.  Using wireless clients next to a cordless phone on the same frequency will cause connection issues.

Hi all - thank you for all your feedbacks.
I am setting up only a very simple wireless network.
I am planning to use Apple Airport exterme for my wireless connectivity.
Could any one suggest is this recommended ?

The Airport should be fine for a small wireless LAN.

I suggest using WPA2-AES security if all clients support it. Otherwise WPA2 [AES+TKIP].
Any clients that support only WEP should upgrade their hardware if a driver update to the latest version doesn't add WPA security.

RajeevSrikant,
Yes the Apple Airport Extreme should be just fine for a simple office environment. As Darr247 mentions you should use security.
Being a simple network I would try to position the Airport centrally and I would switch it from it's default channel if you have other near by wireless networks.
Keep the configuration simple, other than using strong security of course. The nice thing about a wireless network is that it is not too hard to make changes and adjustments to get the most out of your setup, and to accomidate growth.

I have decided in using Cisco Aironet 1250 for setting up my wireless network.
I am planning to order 3 Cisco Aironet 1250 to for my entire office.
From the document its mentioned that it supports 2 modes of frequency 2.4 & 5GHz. I would like to know which is the mode recommended.
Also if i use the 802.1n , i would like to know whether the default wireless client on windows XP will be able to detect the network and can access it ?