A good pair of boots is key to having a good time on the trail. There are a couple choices to be made about the boots you get. First, really is how much of a boot do you want. From what I have learned it appears that just about any boot will do, from light almost sneaker light boots to mountaineering ones. I myself have erred on the heavier boot side of the spectrum, using La Sportiva Glaciers. Overkill for some hikes I do to be sure, but they are not too heavy, and provide great ankle support. Not to mention the boots do an excellent job of both accomidating my orthotics, and keeping my toes from hitting the front of the boot on the way down.
The downside of course is the boots are heavier, which I suppose takes its toll on the way up, though I haven't really noticed it.
Another aspect of choosing the right boot is determining which type you are after, from nylon, to goretex, to full leather boots. I have tried all three and from what I can tell there is little value in having a goretex boot. The auguement is whether there is any added breathability to goretex boots - I never noticed that. I actually, favor a full leather boot, which maintains great water repelancy if treated correctly. Treating the boot is a seaonal type thing for me, and takes only about 10 minutes with nix-wax. You know it is time to re-treat them when you can see the water doesn't bead up and instead soaks into the leather.
As for sizing boots a couple pieces of advise come to mind, first try on the boots at the end of the day when your feet have swollen up to their full size. Remember to wear the socks you would wear when hiking - I use a larger sock expedition treking by smartwool, so this needed to be taken into the equation for the proper fit. Second, try on a lot of boots, not just the one you are thinking of the more the better - it is all about fit. Third, when you find the boot you like best, and size which fits best try on several of them - some will fit better than others. Lastly, make sure when you try on boots that your toes have adequate room. This is done by pushing your feet to the front of an unlaced boot and seeing if you can slip your finger behind your heel - they should fit. Then kick you heel to the back of the boot, lace up and walk around, especially down and incline. Make sure that your toes don't strike the front of the boot, if they do the expereince hiking down hill in them will be uncomfortable.
Hope that helps