Regarding water purification, I think the most important (and cheapest) thing you can do is to acquire and store a few pounds of calcium hypochlorite. Along with storing it properly and saving hardcopy instructions for using it to make bleach, and using that bleach to purify water.
The "Big Berkey" water filters have the best reputation, however you should still test them by filtering some water that is dyed with food coloring. I also think the Ceragrav filters are a very good, lower cost alternative. Finally, the Katadyne filters have some excellent characteristics.
If you do the math, however, the Berkey filters process the most water for the lowest number of dollars.
These guys ( http://www.zenwateronline.com/water-filtration.php ) also look promising for a more limited budget.
There is information on how to use calcium hypochlorite, and various of ways to treat water as well as a good section on various water filters and even instructions on a DIY Berkey filter you can make.
For those who are just starting or are old hands at prepping you may find my Preparedness Manual helpfull. You can download it at:
http://tomeaker.com/kart/Preparedness1j.pdf
NOTE! THIS IS A FREE DOWNLOAD. I DO NOT MAKE ONE CENT OFF MY PREPAREDNESS MANUAL!
For those of you who haven’t started already it’s time to prepare almost past time maybe. You needed to be stocking up on food guns, ammo, basic household supplies like soap, papergoods, cleaning supplies, good sturdy clothes including extra socks, underwear and extra shoes and boots, a extra couple changes of oil and filters for your car, tools, things you buy everyday start buying two and put one up.
As the LDS say “When the emergency is upon us the time for preparedness has past.”
Or as the bible says: A prudent man sees danger and takes refuge, but the simple keep going and suffer for it.
NIV Proverbs 22:3
Lastly this for the doubters and the scoffers.
“There is no greater disaster than to underestimate danger.
Underestimation can be fatal.”