I had a 486 - that's pre-Pentium for those of you who don't know - that I did some upgrading to in the summer of 2000 for a 8th grade student, who had no computer, to use for school papers, etc. No PCI slots, only ISA. I was familiar with PCI video cards and thought perhaps the video in the 486 simply could not be upgraded. It had onboard video. But, I did a bit of research and learned that, indeed, there were such things as ISA video cards. I found one on ebay. It had 2MB of video memory and did, in fact, improve performance of the system.
I have 4 PCI video cards. The oldest has 8MB of memory and is installed in a eMachine 466MHz Celeron that came with, I believe, 4MB onboard video memory. Then, there's a 32MB card installed in an old Pentium. I also have one (64MB) installed in another computer, along with an AGP card, to run dual monitors. The fourth one is not being used now.