Hello Kumar

Firewalls are normally chosen, taking into consideration a lot of important points:

1) no of interfaces needed (Inside, outside, DMZ)

2)Throughput of FW

3) max connections

4) Feature sets needed (IPS etc)

5) Max vpn support..

and many other points.. ALso based on the end user business or use (Datacenter, remote office etc)

Attaching a URL which has comparisons of all the ASA firewalls. Check what you need. You can look out for 5510,20 or 40 kind of appliance.

http://cisco.com/en/US/products/ps6120/prod_models_comparison.html

Hope this helps.. all the best.. happy new year.. rate replies if found useful..

Raj