I would not buy ATT international plan, b/c they're expensive as shit.
Voice:
If all you need is a phone just for calls, you need a phone that's an unlocked international
quad-band GSM phone. If you don't have quad-band phone, you need a GSM phone that covers 900 or 1800 MHz. Most GSM phones you buy here in the US has 1800 MHZ covered.
Internet:
For internet access in Germany and boarder Europe, you need a phone that covers UMTS 2100. So you need your phone to also support
"UMTS 2100" or "UMTS Band 1".
You can find all GSM and UMTS spec data on the back of the box.
Here are all the European networks you can use, just make sure you get one with UMTS 2100 coverage, which is pretty much all providers in Europe:
http://en.wikipedia.org/wiki/List_of_UMTS_networks
In Germany, all German sim providers would work (T-Mobile, Vodafone, O2 Telefonica, E-Plus)
In Italy, you're covered across the board with all providers.
If you're going to France, depending on your US phone, I'm not 100% sure whether you'll get 3G, as I'm not sure if you need a dual channel 900/2100 or just 2100 is sufficient.
Some US phones don't carry UMTS 900 or Band 8, but the iphone5 and 4S do, so if you're using iPhone5, you're good to go in France.
Notes:
Based on the specs of the iPhone 5 and iPhone4S, you're covered for both voice and 3G data across Europe.
You won't get LTE connections however, the world is on completely different bands than USA.