Excellent explanation Havok, though in a A64 system, the CPU gets the info directly from the RAM. The RAM only goes to the northbridge to get the data from the HDD. Thats true for an Intel system, however. Thats also why P4s need DDR2 RAM and such hige caches.
Anyways, Its a moot point. Maxed out, Battlefield 2 can take .9GB to 1.3GB of memory (depending on the map and your texture settings). 1 GB simply doesn't do, even when BF2 is only using .9GB (there are other programs, most notably the OS, running in the background taking up probably 200MB). Thats why its said that 1.5GB is a must for BF2. But because of Dual Channel, people go for the 2GB package (and because they don't make 768MB RAM sticks).
So yea, 2GB IS needed. If you don't have enough memory (AKA. 1GB) then the HDD needs to pick up the slack, and that causes performance loss. Thats where FPS fluctuations come from.