This will tell you how much you can put in. If you use aperture a lot or do image work you should max it out with a total of 6GB especially since you are using DDR2 memory.
As to the question of do you need more it comes down to three factors:
1.) Page-Ins to Page-Outs (if your page-outs exceed the page-ins then you need more memory)
2.) Do you have the money?
3.) What you use the computer for