I was looking for this old topic:
Questions about VLB video ram ?
Unfortunately it seems to lack direct answers to your questions, but it may still be interesting for you.

You need 256K × 4 bit DIP20 DRAM modules. Either 80 or 70ns. Those are obviously different than the 16bit modules already installed so you cannot use chips matching the ones installed.

Speed depends on which MCLK is programmed into the card BIOS. But you should be fine with 70ns.

That's being said, once the 2nd MB is installed, the only difference it is going to give you is 1280 × 1024 / 256 colour and 1024 x 768 x 65k modes a at 43.65Hz interlaced mode (so pretty much useless). I couldn't be bothered installing it.

Considering that this machine will never bo used to display resolutions that high, I guess upgrading it will be rather pointless. Thanks for the advice gentlemen.