Firstly you need to make sure that your Win98 & NT PC's can see any shared folders/Drives on the Win2000 PC's. That your LAN is working correctly, get that sorted then worry about your WAN connections.
I run Win2000 now on all my PC's but used to run a PC with Win98 on the same LAN. The Win2000 PC could see the Win98 machine but not the other way round.
I think the main problem was with the different protocols. Win98 needs Netbuei whereas Win2000 doesn't.
You could try installing the Netbuei protocol on all PC's and see if that makes a difference. Unfortunately I couldn't get it to work for me so went down the Router route - best thing I ever did!
ps I think that's how you spell NetBuei, but I could be wrong!