Yeah, the 2GB more add a big difference in games performance and general performance. With 4GB u also have dual channel that increase the performance of the rig a little more.
Now, for Crysis u need a good amount of RAM (4GB), because is a very heavy game and every day comes out new things that use more RAM on ur rig and with that is better have a good starting point to support the currents and futures games or applications.
Indeed. I only have a 3870, so of course I can't max out crysis with a reasonable framerate. However, with only 2GB, windows complains about lack of memory and crysis will sometime crash or just lag incredibly because of it. with 4GB I have now, I can get 10-15FPS if I max out the settings except for AA. But I still have the same problem if I enable 16x AA. Anyway, 4GB is very quickly becoming a minimum for newer games... and crysis.
Get another single channel 2GB stick. If you keep the computer for the next couple years, you will find yourself upgrading to 8GB.
Even light memory games like MW2 were using close to all of my 2GB of memory before.