It's all about luck. I have an Abit BF6 (BX) which holds a CL TNT2U, it does 89MHz easy, but it will only run at "Normal" AGP, also known as AGP 1x.
I would try reducing the AGP size in the BIOS, and see if you can find a 1x option. If it's still a no go, you're probably better off with a PCI board if you're on BX, since the newer ones will do 1/4 PCI, so if you're on a 133 FSB you'll be 100% safe for the video card.
I'm going with a T-Bird setup shortly, I just hope my TNT2U doesn't die on my by then. 😉 89MHz greatly decreases the life of your AGP card.
Hope that helps,
G|T