It has been a really long time since I posted here. Several parts on my Cannondale 2.8 that I put together in Jan 2007 are showing signs of aging, especially the bottom bracket. I think the bearings are going out. I would like to replace it out. But the problem is, I can't remember what sort of BB this was.

I remember it being English threaded and it's made by Shimano, but can't seem to be able to figure out which one was it exactly. I remember the first figure 68x*** but not the rest of it. Is there a way to figure this out without having to take out the BB?

Start with measuring the axle. This will tell you what you are looking for: 68mm X ??.? Regards the type of bottom-bracket - got a photo? Can you remove it? Is there a bike-shop within 100 miles? Bring it if you are not adept, or have, with using proper tools.

you should be able to find your shimano crankset and bb data listed here
https://techdocs.shimano.com/techdocs/index.jsp

Okay, I figured out that I need 68 x 118.5 english threaded BB. Unfortunately, these things are upwards of $40 plus shipping as they're getting rare now. I saw there is a octalink UN57 or something Shimano BB that is half the price of this one and comes in 118mm. I am wondering how much difference will 0.5mm make? I know I should be getting only the BB specified by the crankset, but what do you guys think?

My crankset is Shimano Ultegra 6503 Triple.

You cannot use the Un57 or something. That is a square taper BB. You have an octalink crankset, and as such your options are quite limited as they stopped producing pretty much everything except the dura ace octalink models. That would be 118 for your triple.

You also have other places to look for used or nos octalink bb's. But they are in quite high demand. $40 for the BB-6500 recommended BB (which is the ultegra version, which is pretty much exactly the same as the current dura ace "track" model) is a huge steal.