From doing lots of AI only tests on identical earth map starting positions and a bunch of actual games I'd throw in a few subjective observations based on the following aggression rankings:
5 = +2 (Germany, Zulu, Mongols)
4 = +1 (9 Civs)
3 = 0 (6 Civs)
2 = -1 (China, Iroquois, Carthage, Korea)
1 = -1 (France, India)
I spent a few weeks with an unpatched version of PTW and no internet access (to dowload the patch), so I decided to spend some of that time testing AI attitudes. This test was with the 1.01 version of PTW, so there might be a possibility that some of this was changed in a patch,
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1) its almost like the Germans and Mongols attract war in a way that leads to their own demise. Perhaps their irritability and aggression translates into threatening other AI to an above average extent, poisoning relations and encouraging a dog pile against themselves? I do not experience this with Zulus and would attribute this to them having a defensive early game UU (see point above about prioritising offensive units) and because they are stuck on a peninsula in southern Africa and so have a lot less scope for war as they are mingling a lot less with others. In general, these 3 Civs never threaten victory in any of my games due to their apparent terrible diplomacy skills. It makes me miss Civ 1 days when high aggression Stalin, Shaka and Ghengis could be relied upon to be the toughest foes in the early to mid game. Whereas in Civ3 they high aggression ones are often the first to die. It might be different if they had an early game offensive UU to compliment that aggression, but they don't.
2) loads of people talk about the Aztecs as aggressive. I've rarely seen it and I've never been concerned by their UU because it is so easy and low cost to avoid war in the early game. There also seems to be no sneak attacks in the early game as the AI doesnt seem to consider this until all land has been colonised and by that time you'll easily repel any Jaguar Warriors. I'd maybe double their cost but give them an attack of 2.
3) Civs like China, Iroquois and Persia absolutely thrive on a consistent basis, and only one of these is Agricultural. Their lack of aggression typically means they are late to join a dog pile and so enter wars with max offensive capacity at a time when their foe has lost its military power from 5+ turns of war. This is a devastating combination and in my experience leads to far more efficient warmongering from lower aggression Civs. I personally dislike this as it seems counterintuitive. Persia in particular is an absolute god at this. It can turtle peacefully, tech at a good rate despite being a smallish Civ (even more so than other scientific civs), then use a tech advantage to efficiently dogpile from late medieval onwards. The Persians and Chinese can come from nowhere to be a contender (whereas Iroquois never fall behind because of Agriculture).
4) High aggression Civs are more prone to fall behind on tech due to prioritisation and getting sidetracked into wars. I also don't like that and it can make their aggression really counterproductive.
5) the AI, as per the link above, does seem to be pre-disposed to hate the leader on score. So dog piles can occur on lower aggression Civs in these circumstances.
6) I would let the Ottoman and Koreans babysit my children. I find them more passive than France and even India. Thinking about it more, I find the Scientific trait may coincide with peacefulness but can't imagine why that would be.
7) no country has reportedly ever won more battles than France since the dawn of documented civilisation. Why on earth are they lowest aggression?
I don't do mods, so my experience and knowledge is limited. But I'd argue the aggression in Civ3 isnt it's best feature. Personally if I was modding I would:
A) marry high aggression to those Civs with early game offensive UUs (e.g. Rome, Persia, Iroquois, although with Agriculture this could be a lethal combo) to try and create a synergy.
B) reduce aggression levels for those with late game UUs or non-offensive UUs. These guys will still get aggressive but you are raising the chances of that aggression coinciding with them having a power spike rather than them looking like an infant throwing a tantrum who then gets dogpiled.
C) prioritise worker building for high aggression Civs with early game offensive UUs. There is nothing worse than being a warmonger that hasn't hooked iron or horses in your territory. Germany is particularly guilty of doing this and being first to be destroyed.
D) the Flintlock patch letting AI use armies is probably a huge variable and arguably the existing flaw that massively undermines high aggression AIs and prevents them snowballing in a way that might help them withstand being victim of the bad sentiment that their above average aggressive diplomacy may cause. I'd argue aggression is fundamentally broken if the AI cannot use military leaders and create armies. This is probably next on my tinker list to see if it makes aggressive militaristic Civs as terrifying as they should be, rather than deeply underwhelming.
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(And there I thought I'd never have an occasion to use that particular emoji!)
= peaceful, 
I've reading o/l articles dismissing this well-known "myth."
