An HD2600XT is a great card for pushing an AGP system in XP, and it can accelerate HD video if you use the right player software. For win98 though it's too new to work as has been said.
For P3 motherboards like you listed, you should probably avoid "bridged" cards (which use a translator chip to make PCI Express GPUs communicate with an AGP slot). Bridged cards have hardware compatibility problems with P3 era chipsets (not sure about the 440BX but it definitely applies to VIA).
So even if you were to install XP to get the HD2600 working, you'd probably run into artifacts when using it in 3D with those motherboards.
I'd love to see a list of what chipsets ATI and nVidia actually tested and qualified their bridged cards to be compatible with. I don't know if such lists were ever released.
The Ti4xxx series and at least higher end FX cards are good if you want something powerful for DX8 and below on Win98. Not sure what's ideal on the ATI side.