If you decide you would rather keep your focus for lodging in Amsterdam, I would suggest something in the following areas as being "walkable" to the main sightseeing areas: 1. Jordaan 2. de Pijp 3. De 9 Straatjes (the Nine Streets) or 4. Amsterdam's Oud West. I stayed in a hotel that was on one of the main canals in between the Jordaan and 9 Streets areas and another hotel that was much more budget in Oud West. Both areas were very convenient for walking just about everywhere and for those places that were further, each neighborhood offered a lot in terms of being able to connect to multiple forms of public transit (trams, Metro, trains to other cities in the Netherlands, etc.).
Amsterdam is not a huge city and the public transit is very efficient and covers many areas. If you find an apartment rental or a hotel/B&B in an area that is a little outside the main tourist zones or not within the main canal rings, as long as you can get to some form of public transit, it's workable. But if you want convenience to walk to sightseeing areas but have that feel of a "neighborhood" in central Amsterdam, those are the 4 areas I would recommend first.
If you can go to the Van Gogh Museum on a Friday evening when they are open late and have lectures, live music and a bar in the lobby, I highly recommend it.
I agree with Harold on trying to book early in Amsterdam if you want more choice, although I will say that I did not book until 2 weeks before my trip and was able to find a number of good deals. Just don't make the mistake some people do and feel like the only places in town to book are those found in the Rick Steves' guidebook, because those fill up early and fast with other readers. Do your homework, read reviews carefully and you will find other options that are equally appealing.
I also agree with Harold that I found Antwerp more interesting as a base although it's not everyone's cup of tea and more people on the board tend to suggest Bruges. Bruges was ok and pretty, but if you are staying in Amsterdam, it could feel like an overload on staying in cities with canals. I also found it a bit dull in Bruges in the evenings - pretty to walk around, but not much going on after hours. I thought the mix of culture, food, fashion, art and nightlife in Antwerp was great. In Brussels, I loved the museum of musical instruments (MIM). Regardless which Belgian city you choose to base in, the others are all easy to reach via commuter rail if you want to take some day trips.