Newbie needs help with crank and bb

I have a Trek 7300 Hybrid (2003) which I've put 7000+ miles on in the last year. Almost exclusively
road or paved bike path. I love the bike, and want to upgrade some of the components. My BB is
giving rattles and groans, and time to be replaced. LBS tells me if I replace crank, I can pretty
much put in any BB I want. Since my cranks are stock Nexave that came with the bike, I'm not opposed
to going this route. LBS recommended Shimano 105 crank, and said I could then do 105, LX or XT BB.
Cost installed about $125 - $150 depending in BB I choose. Would like your views on whic BB. OK, I
admit I know little more than how to push the pedals. Looking online, I guess when you buy new
cranks, you get new chainrings as part of the package? Confusing, cause I see you can buy chainrings
separately. So, you might replace a chainring by itself, but you always replace them when you buy
new cranks?? Next, I understand a double vs triple chainring, but is there an 8 speed triple, vs a 9
speed triple?? Or are they interchangable? My bike is an 8 speed, and I'm not at the point of trying
to move to 9. The current front CR is 48-38-28. The 105 is a 52-42-30. I assume the 52 chainring
will be somewhat bigger in diameter than the 48 I have now to accomodate more teeth. Will I have any
issues with the front DR?? Its also a Nexave, triple front. What will I probably notice with the
changes in gearing? Finally, will I have to replace the chain, or anything else?? Since the 52 CR is
bigger (again I assume) won't that change the length of the chain? Sorry for the long question, but
searching websites is a bit confusing. They assume you know much of what I'm asking. Thanks to
anyone who helps