Not really 'An FPS' Just BF2 really. And think about it - The CPU is used <i>all the time</i>, from some of the graphics operations to Enemy AI and Physics of barrels, particles, and vehicles. A graphics card doesn't know that a wheel can roll. It just knows how to draw one.
If the CPU was really the problem then it'll be constantly straining and going slow, not just here and there.
In BF2, when you spawn, suddenly the surroundings are a bit different to what they were a minute ago, you may have different weapons and the like. If it doesn't have these immediately in memory then it needs to load them from disk - that's slow. If it can't hold every single piece of information ever needed on the level (the map, all the textures, physics and vehicle information, etc) in memory at once, then it'll need to keep swapping out stuff it doesn't need right now to make room. This sounds like your problem.
All that said, 1Gb <i>is</i> still a sizeable chunk, but for one thing windows itself will always consume a little for itself (in fact it'll happily eat 300Mb+ at the desktop), and then you have how efficiently the game is written... Which in BF2's case I wouldn't say is that good. It'll probably get a lot better with patches - don't take my word for it, just do a quick google search for 'battlefield 2 RAM' or something and you should see plenty of confirmation.
You should be able to get a single 1Gb stick and run 2Gb in dual channel (2x512Mb in one channel, and 1x1Gb in the other), but I don't think there's any way to get 1.5Gb in dual channel with 3 Ram slots, although Dual channel really doesn't do a great deal for AMD systems anyway. (5% or so)
And don't worry about 3dmark. You're unlikely to hit 15K in '03 with that card. Check out the VGA charts on toms and you'll see. You'd gain only a few hundred points from an A64 system, a thousand at most probably, as it's so graphics-card oriented it simply doesn't really strain the rest of the system.
Sorry this got a bit long, but I get that way when I'm tired but awake like this...
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