I used to run a 2.4G B (533 fsb) with the old 7000 based AVGA and would get a constants 60fps even on games like Street Fighter EX 2 using Zinc 0.9.
Use the DirectX render (I can give you a screen shot of my general render settings as well if needed - using ZincGui)..
Use native resolutions for games...
For example.... running Street Fighter EX 2 Plus at 392x240 16-bit, I can get 28 frames using that same card right now on my PIII 866Mhz cocktail. Texture features all set to maximum values.
Additionally, Zinc's FPS counter isn't extremely accurate. Playing Grand Master tetris on the same PIII with 7000 AVGA, its tells me I'm only getting 57fps, yet, there is no sound or visual distortion, which implies I am getting full speed as intended as I have the frame rate set to 60. No frame limited gets me about 70-80fps.