Hotdog Zanzibar said:

Yep, you're right... I only looked at them briefly last night. Flipped one over and there's a "168" on the back side. Chips are SEC brand # KM416C1200AJ-6.

Click to expand...

These are 5V FPM 1k refresh 60ns DIMMS.  Each chip is 16 megabits, so an eight chip module would be 16MiB, a sixteen chip would be 32MiB.

These are probably a pull from or parts for a PCI Power Mac/Performa (but not a 4400 [3.3v] or TAM [64MB modules only]).  I expect they'd be compatible with your 6400, although some folks seem to prefer EDO ram in the Alchemy systems.  See: http://www.macgurus.com/products/ram/pcipowermacs.php