This is what i do to increase my framerate :
1: I change my desktop colors down to 16 bits (This helps me atleast)
2: I choose Medium Texture Quality, 16Bits Texture Colors Quality
3: I highlight Compressed textures, then i set my resolution to 1280x1024x16
I have a PIII 800, 256 SDRAM, with a Geforce II 32MB. at minimum framerate in a really crowded area it lays around 10-15FPS
If this dosent help i sometimes, turn down World View Distance.
I also rarely lag if not due to a server issue
And about the Crowd Limiting System, /me likes.. Great job!