If you're riding on roads, then before you go mounting bottle cages everywhere, use google maps or yahoo maps to check for the location of services along your route. Try to find out how far you will have to go between restaurants, convenience stores, etc. If you have a stretch or two where you do need extra water, an expandable water storage like the Dromedary is a good way to go.
A filter is good if you are planning a long trip with very few available services, or an off road trip. But be sure that there are ground sources of water. A filter won't be very useful riding across the desert unless you know where to find a spring. For emergency use, a bottle of iodine tablets should suffice and is cheaper, lighter, and less volume than a filter.