There is a "bug" in the core W9X code that can cause problems with more than 511MB of RAM installed.
The bug is VERY configuration dependant. Some systems work fine with 1GB of RAM.
Others will not boot, or slow to a crawl with 512MB installed.
Benchmark tests have shown "optimum" performance for W98 with 196MB of RAM.
I install no more than 384MB on my W9X boxes.
I would add at least 128MB of RAM, and see what that does.
I assume that your motherboard uses SDRAM DIMMs.
If so, make sure that you can return or exchange whatever memory you buy.
Many older motherboards have limits as to the SDRAM technology that will work in them.
The SDRAM specification has evolved over the years.
Your motherboard may require single density DIMMs, or a specific clock line configuration.
If the major vendors such as Crucial, Kingston, or PNY list your PC, you might want to go with there RAM.
Other causes of slow-downs:
Insufficient free hard drive space.
For virtual memory to function properly, you need a minimum of 512MB of free hard drive space.
You can free up space by deleting files that are over 2 weeks old from the C:\Windows|Temp folder.
And, by clearing your temporary internet files.
IE 6 or DirectX 9 with a video card that does NOT support DirectX 9.
Windows itself.
Over time, the registry and system files grow too large for the PC to function efficiently.
The answer, is to wipe the hard drive, and re-install Windows.
I usually re-install W98 every 2 to 3 years.
If you install a lot of software, you may see a benefit of re-installing W98 every 6 months.
Chas