Most modern machines require at least 512M of "Ram memory" for comfortable use you would need 1G, Computers need memory to run applications and processes, the more memory the better the machine will be able to handle whatever is required.
To find how much memory you have do the following :-
You already know how much ram you have, but just incase open a terminal and type
"free -m" and the result is your memory size in "Megs'.
Get in touch if you think you need memory
Click the "Start" menu the right click on Computer/MyComputer and select properties, if you don't have Computer/MyComputer in the "Start" menu it should be on your "Desktop"
Windows Vista and Windoiws 7 recommends 2G, 4G for 64bit systems. If you use memory intensive apps like Photoshop or Video editing software you may need more.
Select "About This Mac" from the Apple menu and it will display your memory size
10.6 (Snow leopard) or above require a minimum of 1G to run, memory intensive apps like Photoshop or Video/Sound editing software would require at least 2G more likely 4G or 8G.