I've Maxed out 4Gb on my system a couple times. But i do have iTunes up whilst playing games and have various programs up whilst doing digital image work with Photoshop or Corel.
Just think that DDR2 is now Dieing out, DDR is almost Rare and DDR2 is being replaced with DDR3 so When you might want to upgrade you cant as the stock may not be there To buy.