Q: How much more ram can i install if i already have 3gb

Welcome to Apple Support Communities

It depends on the computer you have. To check how much memory you can install, choose your Mac at OWC or Crucial, and buy more memory. Apple provides you the steps to install more RAM > http://support.apple.com/kb/HT1423?viewlocale=en_US

Many people on these forums speak well of the following suppliers.

In North America contact OWC …        Upgrade Apple iMac Memory

In Europe contact Crucial.com  …       RAM Memory Upgrade

Both companies have facilities to guide you and will recommend the correct memory package to meet whatever needs you describe to them.  

There may be benefits buying from your own geographical location insofar as Shipping charges, Local sales taxes and Exchange rates.  Check out the basic memory costs and do your sums.

Guide to required RAM.    Lookup Mac Specs By Serial Number, Order, Model & EMC Number, Model ID @ EveryMac.com

You may find the recommended memory in Apple's guide is less than the above company's offer.  That is not a means to get you to buy more, Apple does tend to be conservative and these people do test extensively and offer a guarantee.