Apple does not sell any version of OS X older than the last two or maybe three, yours not at all.
However, here is one at a very good price that will work on both those Macs.
Those Macs have 4 slots for RAM, you could replace the small one with the 256 and get a small gain but I am not sure how much it will help.
Add them up to get the best mix, paired is better than mixed if possible.
If, once you do this, you get beeps on startup replace the RAM as it was because a read error occurred.
The update at Apple is for a version of Safari that Mac cannot use.
This will upgrade to the latest Safari Tiger can use, do not do this for what you have now.
Yes, reset the Keychain to default, this is where OS X stores all passwords and is tied to the user name and password.
Anytime a password XXXXX XXXXX Keychain needs to be updated also.
There is no back door into the keychain so once locked out it requires a new keychain.
Long road but it is getting there Molly.