Converting from square spline to external bearings, what do I need to know? I need to replace the entire drivetrain on my circa 2000 MTB commuter. Quality groupsets all seem to use external bearings these days. Replacing my current cartridge bottom bracket for a modern external bearing BB is my main compatibility concern. What information do I need from my frame and a prospective groupset to determine if it will fit my bike?

Off the top of my head, I guess I need to know my frame's BB shell width and thread and the chainline. What else? Where do I find the corresponding info for the new parts, particularly Shimano? Anything I need to look out for?



Update:
The bike needs new a new bottom bracket, chainrings, chain, 8speed cassette and front derailleur. Replacing them individually costs almost as much as a complete modern groupset (which includes new levers, rear mech and two more gears).

Why not buy a new bike? Three reasons. One economic: the steel frame is sound, the wheels and brakes are new, and bikes of comparable quality are much more expensive than a new groupset. One philosophical: I don't like throwing things out while they still work. One emotional/irrational: I like my bike, it's been good to me, and I want to do right by it.
You actually guessed the only thing that you really need to watch out for: the BB shell width. And you don't even necessarily need that until you're actually putting it all together. Almost all BB shells are 68 or 73 mm. Most of the external BB's come built for a 73 mm shell and include a spacer for a 68 mm shell. 

The odds are really really good that the threads and diameter coform to the British I.S.O. standard. If you have reason to think that your bike is non-standard, I'd recommend taking it to a shop and they can tell you what you need. 

Unless your mountiain bike is a single speed, you don't need to worry about chainline. They're pretty standard and a derailleur adjustment will take care of any deviation from your current setup. If it is single speed, the BB isn't where you'll be looking to get that right. It's not like a cartridge-style BB where you can just get a different length spindle. Instead, you move the chain rings to the inner or outer side of the spider and/or mess with spacers in the rear wheel.