Unfortunately, a lot of IBM,s have the video integrated into the motherboard, and use some of the system RAM for video as well. This is how they keep the price cheap to compete with better quality un-branded competition. I suspect this may be the case with your machine.
You will need to check the motherboard section of the paperwork that came with the machine, and see if there is a setting to isolate this on-board video. If so, you can then purchase whatever particular card takes your fancy providing you have a spare PCI slot, and the physical room to install it.
Do not get something very expensive in a card. Suggest a cheaper TNT2 chip 3D card, or a Voodoo 3, with min 16Mb RAM. The latest and greatest will not operate at its best due to your CPU (the K6-300) being a little slow anyway.
If I can be of more assistance, e-mail me.
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When all else fails, read the manual!