well the best i can suggest to you, rather than me, or anyone else sitting here trying to give the mile long list of changes, i will just say go to www.kernel.org
and look at the changelog for 2.6 ... your going to have to go there anyways to get the kernel sources for recompiling ;-)
and yes it supports PCI modems, but yours specifically i have no idea ....
i would suggest you look into that, and to get better results use www.google.ca/linux
just see if the kernel supports it, or do you have do download a patch / driver seperate ..
if it works outta the box and was picked up in your slack9.1 install, then it more so is included with 2.6 .. you'll just have to make sure you select it
heres my HOWTO
if you want to reference for you recompiling process ;-)