DOS by itself will only recognize the first 640K of memory.
Foe DOS to use upper memory you need to install a memory manager such as emm386 from config.sys or autoexec.bat.
CAUTION unless you have to run applications from a true DOS prompt (bootinto DOS) DO NOT install emm386 in config or autoexec!
My first suggestion would be to look for updated drivers for the motherboard and video card.
If this is a PC with on-board (built-in) graphics, than a seperate video card will improve performance imensly.
Next, if Windows 98 has been on the PC for over 2 years, then, it would be worth your time to backup all data files and settings,
format the hard drive,
and re-install Windows.
Windows builds up a lot of extra files, and erroneos settings over time.
The simplest way to correct the problems is to start over with a clean fresh installation.
Chas