Xircom works in both Linux and '98, you need Xircom drivers, not Acer or
Phoenix.  As for the 28.8 modem, your guess is as good as mine.  You may
check the Linux Hardware Compatibility List (this is where the laptop
howto/hcl is isn't it?  Somebody correct me if I'm wrong.) at the LDP. 
www.linuxdoc.org.  

* Blake