|MadSci Network: Earth Sciences|
You are correct, the Earth's field does appear to have reversed itself every few hundred thousand years or so. However, this interval is irregular and it appears that there are times when the field has not reveresed for as much as 50 million years, or 500 times the average time. Thus, predicting the next reversal may be difficult at best.
Current trends indicate that the Earth's field has grown 5% weaker int eh past century. If this continues (which isn't at all certain), we may go to zero field in the next few thousand years sometimes, which might lead to a reversal. So you might as well keep your compass for a while, yet.
As for the sun's activity, well, it doesn't really affect Earth's field much directly. Our field in generated deep inside the Earth and us due to moving molten metals, in essence. They are driven mostly by heating within the Earth due to radioactive decays in the rocks and are also affected by Earth's own magnetic field (they field and the moving liquid metals affect each other constantly).
The Sun's activity does affect our magnetosphere, the area around the planet where our field dominates. Different amounts of activity on the Sun can lead to different sizes in our magnetosphere. This is mainly due to the solar wind, the stream of particles blown off of the Sun constantly. When the solar wind is strong, our magnetosphere squishes down under the pressure basically, while when it is weak, out magenetosphere blows back up. We are, however, pretty much unaffect down here on Earth's surface.
If you are curious and want to read some more, you might start with a introductory geology textbook (I have Monroe and Wicander's "Physical Geology" in front of me, and I rather like it). If you want more information on how the solar wind interacts with the magnetosphere, you might try William K. Hartmann's "Moons and Planets".
Try the links in the MadSci Library for more information on Earth Sciences.