Can anyone point me in the direction of a site where I can find all of the old 2nd and 3rd edition hand gestures for the Techniques and Forms? I can just about remember them all but there are a few I'm not sure about.
You can find them here
http://darmont.free.fr/arm/armtut/?lang=eng
as part of Jérôme Darmont's fun Ars Magica tutorial
Tiz
Boy, How-Apprentices-make-mistakes 101.
Senior Master 
Pretty good, I hadn't saw them in ages
How easy are those gesture to make for a magus?
Can you do it while:
- mounted
- tied up
- holding something
- almost unnoticed
- while fighting (with mundane means)
I suspect that the above questions (in practical terms, it is impossible even if the background would imply that they can be performed while holding a staff for example) is the reason why they disappeared into the void of oblivion in ArM5.
And to your information, using them in real life is quite difficult. We used to make players use booming voices andf those gestures if they wanted the casting bonus, and man, we did struggle!! 
Cheers,
Xavi
Now try giving them 15 years of regular practice under the correction of a leather riding crop (or any variety of simulated minor pain effects such as a bic lighter for a handy CrIg reproval) and then see how adept they are at it! 
It will probably end up as a matter of taste, for the troupe to decide then. There are many depictions of magi in movies, pictures and LARP. They offer a wide variety of how physical hand gestures are.
The Star Wars 'magi' obviously cast most 'spells' without sound, but usually have small gestures, only a trained observer will recognise it as magic.
Most witches use almost ritualistic gestures, those will definately attract the attention of everybody observing them.
In the Ad Fons PbP saga, a lot of spells are used in open view. One example is where Yannick uses Revealed flaws of the mortal flesh and some spont. PeCo to make pain, right in front of the grog he is conducting a medical examination on.'
The way my troupe plays, this grog would have ran away in horror, when he feels pain as soon as the magus touches him, certainly after seeing him make strange gestures and uttering mysterious sentences. It does lead to the deft (form) virtue being quite powerfull in our saga.
Yannick has both Quiet Magic*2 and subtle magic
Aha, that explains a lot.
So it is basically a major virtue (3 minors) to cast all your spells without sound/gestures. Except that you can still take another major hermetic virtue... Why didn't I think of that at character creation?
On the other hand, much spell-slinging will be done without troublesome people watching.
I know of one enfeebled apprentice who should be getting that subtle magic thingy 
An option is to take 2 out of free and Flawless Magic, thus saving on one minor virtue by taking the "quiet casting" mastery option.
But then, of course, you will have spent 5 virtue points rather than only 3.
But coming back to my earlier question:
How obvious is casting, when you use normal gestures and voice? And how obvious is it when you only use gestures?
I would say for former, "obvious" and to any mundane "alarming". The latter would merely be "odd" at best or downright "suspicious" if witnessed in conjunction with some evident spell effect.
Im sure you can imagine the relevant respective scenarios per the rumour mill of any village,town, city...
"Yeah, that's the one I saw uttering strange incantation and making odd gestures right before the dead guy over there burst into flames!"
or "I can't be sure but I did see that guy waving his hands about for no apparent reason just before the sky turned dark and lightning began flashing all around."
I have a copy of the hand gestures from a former edition lying around when we play - and the players all take great thrills from using those gestures in their first person playing.
And they've all started out as apprentices -just as newcomers have become their apprentices- and it's been one of many great gimmicks to set the scene and mood.
Even though they are depicted as being used with two hands, I often imagined them being used with one--like sign language, making a little sentence by using two gestures in succession with one hand.
And I on the other hand always liked the idea that one hand is the Seat of the Form and one of the Technique - I havent seen it written anywhere but it's clear from the old drawings.
Oh certainly--just that using one hand allows someone to carry a staff or wand at the same time =D
If you're a true wizard only carrying your Talisman that shouldn't be a problem. But in any case I let my players off with only Suble Gestures penalty when making regular gestures with one hand while carrying something in the other. Or making tentative gesture with the hand holding something should still limit the penalty.
There's already a perfectly sound option in the book if people want their hands to be free: -5 to casting total for no gestures.
Though I do like the idea that large gestures give you +1, so suppressed gestures give you -1, it's probably a lot of on-the-fly storyguiding. (and therefore often forgotten in the heat of the moment, which might lead to a feeling of "the SG is only using this rule to limit my character")