Can’t change video display settings

I’ve made several upgrades to my PC such as:
Soyo SY-5EMA MOTHERBOARD;
AMD K6 500MHZ;
FLOPPY DRIVE;
POWER SUPPLY;
Windows 98

After all of these upgrades everything works fine except I can’t change my display settings from 16 colors. My video card is an STB 128 AGP. I’ve tried reinstalling and reinserting this card but nothing seems to allow me to change the display settings. What can I do to correct this problem?
Thanks.
Len Piko