I'm sorry but i have to agree with Gail on this. The links provided are a good read and an excellent place to start.
In addition Brent Ozar has a video
outlining the things to check if you require more memory.
Also, there are a few good diagnostic queries within Glen Berry's blog
. Query 35, 36, 44, 46 and 50 that will help identify (or point you in the right direction)
If all the advise you have been given appease you/client then I personally would seek the advice of a DB professional on a consultancy basis.