Bicycle Tutor

I figure that I may change/upgrade my crankset this winter.
So I have this crummy junk Tracer crank, real department store crankset. I was looking at getting a double as I find that I don't need the little 22T ring and only occasionally utilize the larger 52T ring. Maybe I could go single but the doubles are pretty reasonable.
I do worry about chain length & chain types or rather which ones to get with a new double.
I have a Shimano megarange in the back, which I like, and would like to keep that in place or get a new one at the least. The megarange seems to have held up pretty well and you can still get the 7 speed cassettes for a decent price.
So what "Gotchas" do I need to watch out for when buying the parts for this upgrade scenario?

You shouldn't have to replace the chain, but you may have to shorten it if you get smaller chainrings on the front. Other than that you'll just have to adjust the front derailleur once the new crank is installed.