The documentation for the IBM Thinkpad 770Z says that it supports up to 320 MB of RAM: two 128 MB SODIMMs, plus the "built-in" 64 MB.
It is actually possible to put 256 MB SODIMMs in the 770Z
memory slots. After doing so, the power-on screen reports
523712 KB OK". Easy-Setup (as the BIOS is called)
says that the system has 589823 KB installed, and 523712 KB
available. I guess this means that the 770Z chipset only
supports 512 MB. (Easy-Setup takes 572 KB of the usable memory:
512 MB = 524288 KB = 523712 KB + 576 KB.)
mem=523712k" on the linux boot command
line allows linux to use all of the usable memory. So far (after
a few months of use), I have had no problems.
I suspect that this would work in a 770X as well, because the systems are very similar. The only differences, I believe, are in the processor speed and the graphics chipset.
I used 256 MB 32Mx64 PC133 SDRAM SODIMMs. I bought them from Crucial Technology on 2001-11-27 for $47.69 each (before taxes, with free 2-day FedEx delivery). The 770Z only needs 66MHz memory, but Crucial doesn't sell 66MHz 256MB SODIMMs. I bought PC133 instead of PC100 to increase the probability that I'll be able to use the memory in my next notebook.
Warning: the memory test in Easy-Setup quits immediately with the cryptic messages "DEV 002 ERR 83 FRU 2010". I have decided to ignore this unless/until I experience problems.