Ok, with assistance of dictionary here:
Obsolete: not in use any more, having been replaced by something newer and better or more fashionable.
Further computer-centric example:
As far as the average user is concerned, the 486 is obsolete. You can't run anything remotely modern on it, you can't upgrade it, nobody (average users) uses them anymore. Nothing is being written for it anymore.
Obsolescent: Becoming obsolete.
Further computer-centric example:
I think it's safe to say everything previous to the Pentium III is obsolescent. Difficult to upgrade, but still possible, and usable by average modern computer standards. New apps can still run on them.