Difference is Dolby Surround is just your vanilla positional audio replicated as close as possible to the original audio source. EAX is positional audio with environmental enhancements that alter the sound field to simulate specific environments. For instance, if in the game you are in a cave and hear a scream from behind you, you would hear a muted scream from the rear channels that might echo carrying into the front speakers, etc. etc.
Chiz