I've never worked with BB's before, and I realize there's a lot of variation when it comes to BB dimensions. I have a windsor knight https://bikesdirect.com/products/windsor/knight_x.htm, which has the "TruVativ Elita GXP GigaXpipe 30/39/52T"

and I'm thinking about putting on an ultegra crankset on to it. But I'm not sure if the frame will be compatible with the 6700 BB.

I'll ask the LBS tomorrow. But I would also like some info from here.

You have English threaded BB shell. You'll need to get a Shimano BB, but compatible for sure.

well the shimano crank came with a BB, so I guess I'm all set? Just take out the old crankset/BB and slip the new one in, it's that easy?

what if I put the ultegra crank on to my new bike with BB30? I've read that I need a spacer for that.

Correct.

You'll need this.

https://wheelsmfg.com/bb-30-shims-shimano.html

so it doesn't matter what the spindle length of the crank is? it just have to be the right diameter?

yes, my kona mtb kula watt carbon come with bb30. im using fsa bb that use for bb ht 2 ass long have the right diameter all working well. spindle length is not concern for bb ht 2, the concern is your frame bb width it come with either 68 or 72 so need chose the right adapter