In GoldenEye, pressing Z while unarmed results in Bond doing a diagonal judo chop. This is not particularly damaging to enemies, and never used in speed strategies.
In Perfect Dark, pressing Z while unarmed causes Jo to deliver a flurry of punches. This, again, isn't especially damaging, unless you manage to catch an enemy from behind without him seeing you, in which case you stand a good chance of knocking him out.
In Perfect Dark it is infinitely more useful to make use of Unarmed's secondary function, "Disarm". This causes you to steal the weapon of whoever you hit, as well as dealing a small amount of damage. This is staggeringly useful as it can be used to effectively neutralise enemies (and gain ammunition and weapons) without having to actually waste time doing battle with and killing them. (Note that this does not work on Skedar.)
|Perfect Dark Firing Range|
Pistols: Falcon 2 / silenced / scoped | Magsec 4 | DY357 Magnum / LX | Mauler | Phoenix