Need some background

What size is current pump?

What diameter/length are the suction and return lines?

Do you have a spa or water feature? How many jets? Run off same pump?

Do you also have a heater and/or solar?

All these things are important when deciding pump size. Also, here are a couple of good sites to help you decide what pump you need:

http://poolplaza.com/pool-pump-sizing-2.shtml
http://www.poolspa.com/publications/...ries/pumps.htm