The Atlas V has many different versions, all built around a common core vehicle. Some of the Lockheed-Martin engineers are calling it "Dial a Rocket" because you can tailor it for all sorts of payloads. Among the more interesting versions, some will include from 1-5 solid-fuel strap-ons and payload shrouds of 4 or 5 meters in diameter and different lengths.

The Atlas V-Heavy is similar to the Delta IV Heavy in that it will have a core vehicle with two more first stages as giant strap-ons. It would be able to replace the trouble-prone and very expensive Titan IV booster, also built by Lockheed-Martin.

Atlas V fact sheet

Atlas V specs

Atlas V Payload Planner's Guide (1.3 MB PDF file)

International Launch Services Atlas V fact sheets