Hi,
So if I understand your question right... You want two internet connections - one through (presumably) your WinXP machine and another one to a Win98 machine to service everyone else.
...and you want to remain on the internal ICS 192.168.. network as well as having your own internet connection (and hence being on another network)
I dont believe that this is a problem You should be able to have the XP on both the networks simultaneously like that.
To set up ICS take a look at the following site...
http://www.internet-sharing.com/
(remove spaces from the above URL)
All PCs will be set up to automatically gain their IP addresses after that. Yours included.
Your internet connection will be on the network connection for the modem this can have a different IP address. Make sure you set the connections to the internet up as dial-up, though. Not LAN otherwise you'll be served by the one 98 machine.
Dont enable routing on your box or you'll potentially opening up your network to the internet.
Also I suggest using a firewall and ensuring every PC has virus protection.
I'd hate for you to be frantically restoring your data in the future thinking "why didn't I..."
Cheers,
Tim.