2x10 pins, so it is a DIP-20 package.
The expansion is to go from 1MB to 2MB, so 8 chips for 1MB means 128KBytes per chip.
This came in 256Kx4 Fast Page Mode DRAM.
My guess would go for example at https://www.ebay.com/itm/303339433779
Datasheet: https://www.westfloridacomponents.com/mm5/gra … 4256A-70PSZ.pdf
That ebay listing is for 60ns. It is on the quicker end. You can check that the 60ns is enough by reading out the exact serial number on those two existing NEC DRAMs in U24 and U25 (can't quite make out the speed rating there).
If they end in -60, -70 or -80, then I would guess the above ebay memory would be good.
If those NEC memory chips end in -50, or -40, then that -60ns eBay listing might not be fast enough.
I have a CL-GD5428 that maxes out at 1MB:
cl-gd5428.jpg
That card uses 70 ns memory.
SNOOP-cl-gd5428.png
Another card, a CL-GD5434 that I have in my lab does have 2MB, comes with 60 ns:
SNOOP_CIRRUS_LOGIC_GD5434.png
So upgrading from 1MB to 2MB will likely provide the 1280*1024@8bpp and 1024*768@15/16bpp. Also 800*600@24bpp should also fit, but depends on the Cirrus firmware if it provides that - looks like my 2MB card doesn't give that, even though it would have enough VRAM.