Youtube has videos of folks running WIN95 on modern 3GHz machines and WIN7 on 386's, etc.
Although I use XP at home and WIN7 at work, WIN98 seems to have had everything I needed. The only problems you run into are missing drivers and internet browser capability.
I am not sure what internet browser capability you lose.
You can go to oldversion.com and get the appropriate version of Firefox for W/98. You can also get the old versions of flash and whatever else you need.
It really depends on what you are doing with that machine. My main machines are on XP with some upgraded products (like Word) but my "appliance" machines are still W98 (weather station, MP3 players etc) mostly because the software I run is not XP friendly but I still want to be able to get to the internet and they all hang on my network.
For the few hardcore DOS users out there, you can run DOS on XP using a free program called DOSBOX but it runs seamlessly on W/98.
