Windows should now have a minimum of 2GB of ram to run efficiently with multiple programs running but you will notice a difference installing 4GB of ram.
Windows 32bit systems ,which most will be on unless you have bought a new machine recently where Windows 64bit is getting more common place now will only use 3GB of ram.
Windows 64bit systems recognise and use RAM from 3GB and up.
So why do i say install 4GB of Ram ?
RAM comes in pairs and the best way to buy is in 2 GB sticks hence the 4GB even though remember Windows 32bit will only recognise the 3GB
The real performance boost I notice is when a Windows XP machine goes from 512MB of ram to 2 GB of ram or in a Vista machine when installing the 4GB.
Windows 7 runs exceptionally well with the 4GB of RAM.
For the extra money , $150 on average it will make all the difference.
The same applies for Mac's with most sold with a minimum of 2GB installed but Macbook Pro's run well with at least 4GB in them.( Check your model if you have an earlier Macbook Pro as earlier models do not except more than 2GB of RAM )