I recently replaced a generic pair of 512MB PC3200 400MHz DDRs in my PC with some nice new Corsairs. My parents asked if they could have the old memory to use in their PC. I know their machine is pretty old; I'm not sure what the mobo make/model is, but I know the CPU is an Athlon 800 Mhz and is AT LEAST 5-6 years old.

The question: assuming my old DDRs fit (184 pins and all that), can I run faster memory on an old mobo, or is that a good recipe for a house fire? Should the memory be able to slow down to suit the old system? Do I need to adjust any timings or anything?

Sorry for the lack of information; I'm at work right now and I haven't been by my parents' place to really investigate their setup yet.

Thanks!

Memory will slow down to match the FSB just fine. So you can run 400MHz memory on a 266MHz board.
Be sure to check the number of notches in the old memory with the stick from your computer before attempting to put it in.

Terrific; thanks!