I'm not an expert, but I'll take a shot at this.
The 170mm dimension is the length of each crank arm, from the pedal axis to the crankset axis. Less than 165mm crank arms are VERY short and I doubt you need or want to go this short. Here are some guidelines I've seen published:
under 5'.......<=70 cm........165
5' to 5'2".......70-74 cm......167.5
5'2" to 5'7"...74-80 cm......170
5'7" to 6'..... .80-86 cm......172.5
6' to 6'5".......86-93 cm......175
over 6'5".......93-99 cm......177.5 - 180.0
I've never seen crank arms available for less than 165mm and 170mm is often the shortest you can get in commonly available cranksets. And I wouldn't bother replacing your crank to go from 170mm to 165mm - not worth the expense and trouble. Most don't think crankarm length is a critical thing anyway if it is within reasonable limits - it is much more important to get other sizing things right (frame size, seat height, stem length, etc.). Your money might be much better spent just gettting a good fitting done on your current hardware.
A new crank with shorter arms than 170mm may be available that is compatible with your bottom bracket (the bearing assembly that threads into your frame, although you may have to hunt around. If you're interested in used parts, you might just cruise Ebay and Craigslist for awhile looking for someone sell an old crankset or even the complete bike which has the short arms you're looking for. If the bike has the same style crankarm attachment (typically a square spindle), then you can swap the crank and leave the BB alone, although you may have to use other chainrings and certainly will probably have to make derailleur adjustments.
Cotterless generally refers to a crankset that attaches to the BB by a press fit onto a square tapered spindle. The spindle is typically part of a 3-piece BB with two cups that thread into the frame's BB shell, the steel spindle, and loose ball bearings.
The key frame spec for swapping BB's is the dimensions and threading of the BB shell that the BB cups threads into. The most common is 68mm and English threading. To change out the BB, you have to figure out the dimensions and threading of the shell and be sure you get the right one. There are some other considerations like spindle length and chainline to worry about later, but the BB physically won't tread in unless you match the width and threads spec.
You can often substitute a new-style cartridge BB if your BB shell is a standard size. If you do end up swaping your BB and crankset, you should probably let the LBS take care of it for you as it gets complicated, requires special tools, and removing your old BB may be VERY difficult.
The "52/42" refers to the number of teeth on the front chainrings. If you get a new crank, you may have to buy new chainrings and possibly have different gearing.
Again, there may be good reasons you want to upgrade your BB and crank, but I don't think shorter arms is probably worth the trouble, assuming you can even find them.
This might be helpful:
And the sheldon brown site has a lot of good info on cranks and BB's also.
Hold your horses: If these are still available and you have a standard 3-piece BB with square tapers on the spindles, this might bolt right up and for $80, you'd have a new crank, new rings that match your current, and 160mm arms. This would seem to be a great way to go: