I would imagine that you can only buy the upgrade disk 2nd hand now, as Win 98 has long been discontinued.
IIRC, Win 98 was little more resource hungry than 95. Certainly a pentium 133 with 32 mb RAM was fine, and I certainly ran it on a 486 dx4 100 with 32 MB RAM.
If possible, try to get Win 98 SE, as this has better support for USB - among other things.
I definitely recall upgrading Win 95 to the beta Win 98 and I had no problems.
I ran the beta code for almost 2 years, then I got the full 98SE which ran for another couple of years until I bought a new PC.
Ususal caveats apply - back up all your important data, so if anything goes wrong you have a plan B. If you still have the distribution media for Win 95 that would be useful!
SD