The Yahoo Rainwater Harvesting Forum has a number of knowledgeable people and success stories, as well as many resources (references, spreadsheets, etc) in the Files section. You must join the Forum to access the information, but membership is free.
I modified the Rainwater Harvesting spreadsheet from the Forum to accept a rainfall threshold in the irrigation calculations and to not have negative amounts in the tank if water needed exceeds water available. download
Drip irrigation supplies are usually available both locally (home centers, garden supply stores) and from online suppliers. Dripworks has a good selection, including some drip emitters that are rated for use with a low pressure source.
The rule of thumb for watering plants is that 0.68 gallon per square foot equals one inch of rain and plants usually do best when thoroughly watered (one inch of rain) once a week. Measure the length and width of the area to be watered, then multiply those values to get the square footage. Example: a flower bed is 6 feet wide and 10 feet long. 6 times 10 is 60, so the area to be watered is 60 square feet. For one inch of rain on that area, multiply the area (60 sq ft) by 0.68 (gallons per sq ft for one inch of water). 60 times 0.68 = 40.8 gallons (use 41 in calculations) to provide the equivalent of one inch of rain. This means that you will need to provide 41 gallons 4.3 times a month (4.3 weeks in a month). 4.3 times 41 is 176.3 gallons each month during the growing season (less the actual rainfall). The spreadsheet allows you to specify how much water you will need for a month and the rainfall level that indicates you don't need to water. This isn't an exact science, as the spreadsheet can't detect if all the rain comes in one weekend and the plants need watering the next week - that requires either human intervention or a sophisticated irrigation system that can detect the exact amount of water in the soil (expensive, at several hundred dollars per detector).
Unless you have an incredible screen system on your rain gutters, the rainwater container you use needs a leaf/trash filter. The barrels you see with an attached flexible downspout connector may look good, but anything that gets into the rain gutters will get into the container.
Openings in the tank (inlet, overflow, etc.) need insect screening in areas where mosquitoes may breed (most of the southeastern US).
Beware claims that white barrels "make it easy to see the water level". White barrels/tanks allow enough sunllight in to promote the growth of algae, unless you regularly add a small amount of chlorine bleach to the container (think you'll remember to do this every time it rains?) I paint all barrels and tanks, even if your color choice is white, to avoid this problem. You can remove the plug in the top of the barrel to check the water level, either by eye or using a long stick (yardstick is ideal).