When using a smaller amount of memory than is needed by a game, your CPU has to Ram the Hard Drive (no pun intended) while using PageFile memory to get the additional data that can't fit into the memory. That leads to the stutters and drops in FPS that you see in the chart above. A miserable experience.
Just keep in mind that more RAM only helps you if it's used. Consider a situation in which you have programs that take up 500MB of memory, plus the 256MB that Windows takes up. In such a case, you would get the same performance from 1GB of memory as you would from 2GB. So it does really depend on the programs. Also keep in mind that games take up more memory, the higher you turn up the settings. I believe BF2 takes "only" 400MB at low settings, and over 1.2GB on High.
well depends all my are fine with 1 gig only 1 game jumps around during the fisrt 1-2 mins after that is fine battlefield 2, my other games are fine. and infomatic the chart you showed compares 512 to 2 gig. i wont consider 512 mb for any gaming machine today. so 1 gig is fine, stay with that 1 gig until more games come out, that you play, that needs that other 1 gig.