Usually the reason for adding more switches to a network is to add port density or increase the reach of the network. Adding switches will not typically increase network performance unless you are replacing old 10/100 switches with 100/1000 switches and doing etherchannel between the switches to provide higher bandwidth between them.
To add to the above answer, understand that the speed of your network is only as fast as the slowest component. This is to say that if you do add faster switches but are not upgrading node NIC(s) to that same 100/1000 switch, then a GB switch is uselss.
(WS) Similarly, consider your network topology when adding more switches. If you have 4 floors and the main switch which goes to your servers is on the first floor, attempt to wire the 4th floor switch directly to the 1st floor switch (GB) and not daisy chain through switches 2 and 3. To attempt to clarify the issue: a 4th floor workstation user wants to save a 2 GB file to the server. Poor network implementation would force this through the 4th floor switch, then through the 3rd floor switch, then through the 2nd floor switch, then through the 1st floor switch and finally into the server room. Not only would this file have to pushed through several switches, it would also be slowed down by any other information being pushed from floors 1, 2, or 3 because the information is being bottlenecked into one connection. Better network implementation would have a GB link from the 4th floor directly to the 1st floor so the information follows the path of least resistance. For you main backbone you should have all GB because that is where traffic is the heaviest.