OK, regarding that ol' 2501 that I couldn't load IOS 12.2 or higher unto; due to TFTP checksum errors. I've bandied the two 8MB Flash SIMMs about until I ascertained that one of them was causing problems (wouldn't even load IOS 12.0(8) (got checksum error even then), then I set it aside and installed the other SIMM in Slot 0; IOS 12.0(8) loaded fine).
I had a third, as-yet-unused 8MB SIMM lying around, and I DO want to have 16MB Flash so I can load 12.3 and do IPv6 routing, so I installed that third SIMM in Slot 1. Now at boot-up I get this message:
System Bootstrap, Version 4.14(9.1), SOFTWARE
Copyright (c) 1986-1994 by cisco Systems
2500 processor with 4096 Kbytes of main memory
% Mismatched chip sizes (size1=0x200000, size2=0x100000)
% System flash query failed. Access will be RD-ONLY
What's with them 'smatched chip sizes? Did I just buy a batch of zotzed SIMM's, or what? ($11.99 each on the eBay; but is that really so below-market considering the age of the 2500 series by now?)
I tell you I'm nearly scared to try anything else; for all I know I'll go back to the land of unrelenting checksum errors....