I just fired up a Falcon 4 campaign to check the numbers. This is what I got:
Falcon is using 140mb of my 512mb memory, and the Task Manager tells me that there are 205mb left that are free.
So Falcon 4 and the OS alone is using about 307mb of the memory. This means that when you have only 256mb that 50mb have to be swaped to the slow HD instead of beeing in the fast RAM.
This swaping is mostly recognized in stuttering because the program is waiting for the data to be read from the slow HD instead of the RAM.
You don't get more fps in F4 from more RAM unless you have a really low amount of RAM and you get constant stuttering from that.
But a GF4 wouldn't give you much more fps than your GF3 either.
A 1.8gHz Athlon with 512mb RAM and a GF3 should normally be enough to run Falcon 4 "smooth".
Of course not with max settings, 1600x1200, 4xFSAA and a campaign running. You better get a Voodoo5 for FSAA
Compared to other up to date games Falcon 4 is depending much more on RAM and especially processor power than on the features of the graphics card simply because it doesn't use this features which all the other games are using so they can afford to look much better because the graphics card is doing all the work.