Most Atlanta gardening books state the last frost date is April 15th. The Atlanta area is still prone to cold weather and perhaps frost before than.

I know Pike Nursery already is selling a wide and beautiful array of annuals, and it is difficult to resist the urge to buy them, especially as the weather is so nice and warm, but it is still too early yet to plant them. Wait till the nights are warmer and there is no more danger of frost in the weather forecasts before you buy and plant begonias.

Infernal Tax Day weekend would be a good time to buy and plant begonias and other annual flowers.

appeared to have been planted for a while because of their fullness

And perhaps because the previous owner fertilized them a lot. I am a terribly poor fertilizer because I am forgetful, busy and cheap! But if you are better than I am, you can have robust looking annuals by fertilizing them with both an initial application of slow release fertilizer (like Osmocote) and regular applications of water soluble stuff like Miracle Gro. Don't overdo it, of course!

Here is a link that might be useful: A nice overview of fertilizing annuals/perennials

My guess is he could have started with larger plants, or did he have a greenhouse? Maybe they were over wintered in a greenhouse.