Hi:
Welcome to Atlanta! You'll be living very close to me, and you will have a wonderful resource near by to help you with your gardening (well two, actually). Dekalb Master Gardeners are quite active, and are very helpful (two live in my neighborhood) and also Oakhurst Gardens is in Decatur and can help you with all sorts of things (and they have classes).
You can a) ammend like crazy, and get your soil going, make raised beds and work above the clay line (works fast for veggies and annuals), and if need be, you can install French Drains but only if you've got the lake or puddles that just won't drain.
There are several places that can help you install rain water irrigation systems underground. I am in the process of finishing our underground system but I also use rain barrels. Cisterns are expensive to install. No doubt about it. Each of my cisterns is $1500 gallons and costs $600 each. There's a store here that sells everything you need for them. Then you'd include an irrigation system if you'd like, and you'd need to hop on an extra filtration system (between $300-$600). I have no idea what sprinkler type systems cost (to install or to run). However all your water will be free. Inground tanks can be installed where there are few trees - if you are treed you can get an above ground tank installed and still outfitted to water automatically. You can even have this powered by solar if you choose! Another option is the Rain Water Pillow which was invented by someone in Georgia. It's a giant space age pillow that sits in your crawl space and fills from your gutters and is attached to your watering system. My neighbor has one. It's wonderful. I've also had some clients have them installed. There is a rain barrel company here that will customize your barrels and make stands or whatever you need (one of my clients has a tower of them!) and paints them to match your home, and comes to install them. These can be made to work with soaker hoses if installed very carefully (which is where the towers come in) and are very reasonably priced.

Thanks. I'm so glad you included approximate costs as I was thinking a rain harvesting system would be significantly more expensive. The the information about the master gardeners is exciting. When I first made this decision, I thought I'll be sad to leave So Cal, but the gardening and house buying prospects just make it better and better. Well, maybe I should say house buying and gardening...;)

I did not include labor. That is, of course very expensive in many cases.

There is a man in Decatur very close to Oakhurst Gardens who has several rain catchment and automated watering systems in his garden. He also hires someone to clean his tanks annually. Here's a company (I am not saying I recommend them, I don't know this company nor this person) that does this locally http://www.raincatchers.net/

Thanks so much for the information - I really want to create the most energy efficient house I can and have a plan for that, and, because I plan to garden in a big way, water has to be a primary consideration. I was worried such a system would be cost prohibitive, but you've reassured me that some extra savings now will allow me to install a water harvesting system.

I read the website you listed and also www.ecovieenvironment.com - of course more research too.

Now I just have to hope for hiring instead of laying off teachers in the area.

Welcome to Decatur from another originally West Coaster, I moved into the city of Decatur last year and I have a tiny lot so I haven't done much more than build a single raised bed for veggies in the only sunny spot of my very shady yard and transplant some hostas from my mom's garden in Seattle. Funny enough, except for the very hot summers here the plant zones and winter weather are very similar here to the Pacific Northwest. From the reading I've done the best method to deal with the clay soil (from a long term perspective) is to just keep mulching. If you get enough organic material on top of the soil the worms come and help aerate for you. So far this is working okay in the beds but my grass is still struggling. I hope by aerating and reseeding in the spring I can actually have some grass and not bare clay in my front yard.

caeebe, you need to get the BEST grass for your exposure. There are many great new grasses available now for the Southeast. There are separate types for sun and shade and part shade and there are some which are simply better by planting plugs or laying sod.
I like SmartSeed
I think you are wise to take it slowly and get a handle on your new yard and the lay of the land. I moved here from Canada and it took me a year or two of experimenting to see what this property needed and what I had to learn (A whole Lot!!!).
Winters are somewhat similar to the NW but we definately get less rain - our rain patterns are different too. You'll begin to notice that we get rain all at once. Like a giant bucket just dumping rain most of the time and not as many "rainy days" like they get in the NW and even in Toronto where I am from. Then of course we go through the DRY hot periods which are so unlike the PNW.. :) Spring and Fall can be very similar though, especially in a good, long spring year!

I wish you gardening success!

GGG

ae2ga,

you were talking about above ground rainwater harvesting, this is something that is being used in asia. Its a wall mounted sorta system like the air con compressor.

do a search on 'voda rainwater harvesting'.

Depending upon where you move to in "Decatur" (which is both a city and a much larger mailing area in unincorporated DeKalb County), your need for amending soil and constructing raised gardens will change. Many older homes in the area have or had gardens, so you might be surprised at how productive the soil is. Water usage will also change based on shade, gradients, and soil conditions; among other considerations. Water catchment equipment will be useful regardless, but the actual size needed will differ as will the number of catchment vessels needed due to the specific layout of the lot (Do you only have room for one large container or is there room for several smaller ones connected in series?) The dought has moderated somewhat from a few years ago, but xeriscaping is popular. In addition, new drought-resistant grass cultivars have been developed to reduce lawn watering needs. This just scratches the surface, but my basic message is that the north-central Georgia climate can be adapted to fairly easily with some work and patience.