I don't believe Steam would be supported. Trying to recall, but I'm pretty sure it supported Windows XP at launch, possibly Windows 2000. (2002 was the release date for Steam, and they had begun development 2+ years earlier, likely with Windows 2000 (NT 5))
Win98 was fairly common back then, but not so much with gamers as to keep up with Direct X you really needed to be on the latest Windows platform, just like today.
Also a very turbulent time for GPUs. It was around then that a lot of vendors started merging or disappearing. And driver support slowly started slipping away.
Your modern hardware would not have drivers for such old OS. You wouldn't be able to use many of the contemporary browsers. A lot of applications will rely on certain windows modules and .NET frameworks which don't exist in Win98.
Then you have 64bit. No such thing as Win98 64bit, so many modern titles would be impossible to run.