depends on a lot of factors I think. Like:
1. What kind of programs will you be using?
2. How much do you really multitask.
3. And operating system
I just built a rig, and I went ahead and invested in 4GB of RAM because I know what i'll be doing. I like to run Photoshop, Flash, AfterEffects, Dreamweaver, multiple instances of web browsers, iTunes, AIM, etc. all at once.
Better to have more now imho.