Originally Posted by Holy Smoke
System RAM has little or nothing to do with it but video RAM makes a somewhat big difference. You should try putting shadows on lowest and turning off HBAO as those are the biggest overall performance killers by far. The shadows eat into your video memory and fillrate while HBAO takes shader performance. Those two make an average difference of ~40fps vs. 60-70fps on the GTX260 in my system. AA makes almost no difference on the GTX260 whereas it made a big difference on my 8800GTS, suggesting that memory could be the bottleneck. Since your friend has a 1gb card then all of these combined could definitely account for the difference to your 512mb card.
Your mate isn't explicitly telling an untruth because performance in the game wavers immensely depending on how much is going on. He could quite possibly be getting a fluid framerate in a 16 player match (though 80-90fps is an exaggerration) at those settings.
You're right on many points... but you'd be surprised how big of an effect system RAM has on the battlefield series. I haven't tested with 4gb, but its very possible given the right system conditions to make the difference between 4gb and 8gb noticeable.... but rare
I originally had a P4 (3ghz), 1gb of DDR400 and a 6800 GT (ultra speeds). It was a hell of a system at the time. BF2 came out and I gave it a shot. I had horrible performance. Unacceptable at the resolutions I wanted to play at. Anytime I cranked it up to 1600x1200 it'd choke to like 11 fps. I borrowed a friend's 1gb and threw it in my rig for 2gb and I was running smooth at 50 fps at the same settings.... it mattered, alot.
This difference you're seeing between 4gb and 8gb though would be a hard one to hit because you'd have to be running very close to the 4gb limit (3.3gb or so) before you load BC2 for it to choke with 4gb.