Obviously when your health is 100 you can't pick up any more (normal) health, but you can add armour. But do they act different against damage? Or is armour just extended health?
When you've hit with armour, you do lose health too. Would it be better then to have 80 health, or like 50 health with 20 armour ? (wild value guesses)
Quake3World has an extensive guide to how armour and health work:
When you are shot, Quake3 applies 2/3 of total damage to armor ..., then the rest is taken off health. If your armor is insufficient to cover the 'debt' the leftover damage comes from your health
So when you take damage it takes 2/3 of the total and applies it to your armour. It then takes the remaining 1/3 plus any left over if your armour was totally destroyed and applies that to your health.
If that takes your health to below 0, you die.
For example, if you have 100 health and 20 armour you can take 120 damage before you die; if you have 100 armour and 20 health, however, 60 damage is enough to frag you - 2/3 of 60 is 40 which comes off your 100 armour but that still leaves 20 damage to come off your health. As that's only at 20 you die.
The function of armour is to reduce the amount of health you lose when you get shot etc. In terms of benefit any armour is better than none, and the stronger your armour the better as it means that none of the 2/3 "armour" damage will get applied to your health.
Quake 3 (and Quake IV) behave as described by ChrisF; armor protects you from a fixed 2/3 of incoming damage, assuming you have enough armor points to absorb the damage. (If you run out of health, you die, regardless of if you have any armor left or not.)
For the earlier games, the color of the armor you pick up matters too.
In Quake I and Quake II, green/Jacket armor provides 30% protection. Yellow/combat armor is 60%, and red/body armor is 80%. Armor shards from Quake II will increase your armor points, and the armor will be of whatever color you currently have; if you don't have ANY armor when you collect a shard, the points behave as if you had green armor.
The Energy Armor/Power Shield from Quake II provides 2/3 damage protection as well, but it's an item you switch on and off, and it uses your cell ammo instead of being a separate resource.
