That's the biggest data package AT&T offer, and 800MB is a lot of data, so you could probably go with the lowest package of $30 for 25mb if you just want to use it for looking up directions/checking email etc.
If you want that amount of data, that's probably the simplest way of doing things. There is no main carrier that can be used across multiple countries in the Europe as the EU has imposed regulations on it.
Getting a sim card in each country would work, and would most likely end up cheaper as there are some pretty good deals out there, mostly with big carriers like vodafone, but would be far more complex, time consuming and annoying depending on how long you're spending in each country.
There are a few other options, Vodafone UK offers a pay as you go european travel option, which should cover all the countries you're going to but it's not a ton of data. Vodafone Data Traveller is £2 a day ($3.10) for up to 25MB Data and 45pence a Mb after that.
The Tep Wireless pocket wifi is pretty cool, definitely worth considering.
http://www.tepwireless.com/tep-me/tep_wifi_countries.php#a_product_features
Oh, and make sure you're phone is unlocked, and if you plan to get a sim in an individual country and you have an iPhone 4 or above remember that especially with language barriers the smaller sim's can be difficult to get!
Hope i've helped. Good luck on your travels!