You can use the same ISP as you are using for the Windows 98 connection. All ISPs these days use PPP for connections, and the remote server is only interested in the fact that something on your PC can request a PPP connection, and then use that connection. The OS does not matter.
In Windows dial-up notworking for your ISP, note down the phone number, and your login name and password. Then boot into Linux and start up "kppp". (Even if you are using Gnome, kppp will still run.)
Set up a new account using the details you have noted down, and then try dialling. If you are unsure as to what to enter in a field, move the pointer over the field and click the right mouse button for context sensitive help.) You should find that you can connect pretty much first time. If you have problems connecting, check for errors in /var/log/messages. You might also want to dial the ISP with kppps log window open.
(For UK folks reading this, I connect to my (currently) toll free LineOne account using kppp.)
Having said all this, if you are in the UK and want to use an ISP that feeds money and resources back into the Linux community, I can highly recommend UKLinux at
Access is via an 0845 local rate number.
And from the home page: "And remember, all the profits from uklinux.net go to funding Open Source/Free Software." This is one excellent ISP!
Hope this helps.