With the bulk Windows Vista carries and the ability to run Crysis, it takes up a lot of system resources. RAM helps to speed things up in your case. But I wouldn't go anymore than 4GB. For the typical user, I wouldn't recommend that much RAM since they won't see a difference in speed at all (if all they plan to do is word processing, internet, email, things like that). Just running Crysis alone is intense enough. I couldn't run it on my desktop PC, but then again, look at how ancient that sucker is. It's not even dual core and the RAM count is at its bare minimum.
Make sure you check your RAM specs before you buy and upgrade them. That program CPU-Z tells you that too. Just go to the SPD tab at the top and it will tell you what type of RAM you're running so you can match it.