Don't go for higher GF cards than the FX-5XXX series when running Win9X.
The 6XXX series are for WinXP and higher. It's all driver issues and lack of technology/features.
Radeon9600/9800's give a better result than GF-6600/6800.
Then there are cards like V5-5500, V3-3500, GF4-ti4600..... Just search Vogons.
There are a lot of info here. Personally, I go for Radeon cards if high performance is needed.

Oh, sorry, I have never used those cards on 89MHz AGP. I'm VERY conservative about OCing old hardware (which is why I favour VIA chipsets for 133MHz Coppermines/Tualatins when I want ISA slots). All I was refering to was the 640x480 limitation within Windows. Then again, if Geforce4 Ti's are fine with the OCed bus, I'd have a hard time believing the FX would be any worse (it seems they inherit the same AGP implementation).

Those refresh rate issues I mentioned were with the FX 5200 my Athlon 64 system came with (I have sold that vídeo card years ago when I upgraded it). I remember researching back then and reading that nVidia disabled refresh rate control in their VESA implementation (fixing all modes to 60Hz) to avoid incompatibilities with LCDs.

Oh, and do search for the equivalente Quadros. Could not find a good GeForce 4 Ti 4600+ here in Brazil when I wanted one, but the Quadro 980XL was readily available for a reasonable price.