Bicycle Tutor

I currently have an Octalink triple crank and the bb is 68x121. I want to replace the octalink crank with something else, most likely a truvativ elita triple. I can't find any other bottom bracket that is 121mm wide. The Truvativ mentions 118mm which is the widest I have been able to find. Will this work or am I stuck with this octalink for the rest of bikes life ( god I hope not) ?

I'd say yes. The 121mm is the axle width. It is only important that the crank set matches this measure, otherwise the chain line is wrong and the front derailleur will not work.
Look also at
http://www.sheldonbrown.com/gloss_bo-z.html#bottom
text below the tables for a better explanation.