Talk to me about Bottom Brackets?

[Susan]

Normally I am quite able to service my bike on my own but this is totally new for me:

I'd like to switch the cranks and bottom bracket on my going-to-town-bike. My plan is to go from a double to a single chainring, 'cause I just don't use the smaller one at all. Currently, there is a "Truvativ"-System bottom bracket installed (the kind normally used on a mountain bike).
So, as the bike is a Soma-mixte, I'd like to switch to a more old-school look for the cranks (something like those http://www.befixed.de/Kurbeln/Track-...-Set::778.html) - and for those, when I got it right, I would have to switch the bottom bracket too. I figured out that I should use a "JIS" bottom bracket, 113 mm long.

When I look for JIS-bottom brackets, there are two standards: "BSA" and "ITA" (which stands for british and italian?). The threading on those two are different.

Ok... how do I find out which one I should get? I would have thought that something like that should be easy to find out by looking at the bike manufacturers site, but... not really... it says:
"Bottom bracket: 68mm"

Is it true that this means its JIS ITA (read this as a side note on another discussion regarding bottom brackets...)?
Is there a way to find out by looking at my bike?

I feel like there is a secret conspiracy to prevent ordinary people from finding out the dirty secrets about bottom bracket standards why is this soooo complicated?

[Sky King]

Read this blog post by Jan Heine BB Demystified Think it will be helpful.

[brigada]

I'm pretty sure Sheldon Brown will provide answers, have a look at his website

[Susan]

Thank you a lot. Sky King, wow, that was really helpful, I knew it wouldn't be that complicated when explained right
Sheldon Brown provides a lot of information too but I just could't get to the bottom of what he was trying to tell me

So I'm pretty sure the Soma uses the british standard, but I can just check the threading once I remove the old BB.