Something here didn't make sense - so I went through the trouble to download the ESC manual. And sure enough, there it is on page 8 of 66:
Supports 256-Mb, 512-Mb and 1-Gb DDR technologies for x8 and x16 devices
Supports up to four unbuffered DIMM
* 915P chipset can only support 256-Mb, 512-Mb and 1-Gb DDR technologies for x8 and x16 device, NOT support 128-Mb DDR technology. That is, 256 MB Double Side Memory Module & 128MB Single Side Memory Module are NOT support."
This above it written verbatim - all typos, spelling, capitalization, punctuation, etc. is exactly as it is in the manual.
This doesn't make any sense to me at all.
This motherboard has one interesting odd thing with regards to video cards and that is that it has both an AGP-e slot and a PCI-e slot. I've never heard of an AGP-e slot, so this is a new thing to me.
As far as the original question, I am completely confused as to why the motherboard should care at all what the amount of RAM on the video card is. It's almost like the authors of the ESC manual are confusing two different things: memory organization on the DIMM itself, and PCI-e onboard memory.
I can understand a motherboard not supporting 8Mbit or 16Mbit memory chips on the DIMM itself - these are old technologies. So that would eliminate 64MB and 128MB single-sided DIMMS, and 128MB and 256MB double-sided DIMMs.