For a thorough discussion of water in the wilds, I'd refer you to Colin Fletcher & Chip Rawlings [/u]The Complete Walker IV[u] and Ryan Jordan's [/u]Lightweight Backpacking and Camping[u] (Alan Dixon and George Cole are listed as co-authors on some editions of Jordan's book.)

I can't speak with any authority on filtering in the western US. In the eastern US, filtering with a .2 micron filter is generally adequate, and no additional treatment or boiling is needed. "Filters" are labeled as "filter" or "purifier," indicating (to oversimplify) whether it will kill viruses; both will kill most other bugs. Because harmful viruses are not commonly found in US water (yet), filters and purifiers are functionally equivalent in the US without additional chemical treatment, and chemical treatments are generally effective without filtering.

However, you need to make some inquiries with the local management agency where you'll be hiking (US Forest Service, Park Service, or state equivalents.) They'll be able to tell you whether there are any local conditions that affect the sweeping overgeneralization I made above. For example, there is a cyst at Isle Royale that chemical treatment will not kill. Your only options there are to boil all water, or use a filter with a .2 micron absolute pore size or smaller. Likewise, there are places in southern Ohio where the tailings from old mines have so poisoned the water that the concentrated chemicals (such as arsenic) cannot be neutralized by filtering, boiling, or chemical treatment. Your only option then is to carry all the water you need. (At one popular spot, the Ohio DNR actually buried water tanks, and keeps them filled with potable water for hikers.)

Read one (or both) of the above books, and supplement with local knowledge, and you'll be fine.