As Flap Sup said, the 'E' and 'X' both have the higher MTOW compared to the base model, and this is due to the fitment of a centreline fuel tank, larger vertical stabiliser, enhanced and strengthened wing. The subtle difference is that the "X" models are designed specifically for longer range and higher MTOW, while the "E" models also feature advanced cockpits/systems which the "X" models may lack.
Originally people referred what is now known as the A340-300X as the "A340-300E" before Airbus officially released it with the "X" designator, causing some confusion. Then the "Enhanced" A340-300X was announced with superior avionics, new and more FBW controls, new CFM56-5C/P engines, and some other details. Swiss' A340-300 is one of these "Enhanced" models, although I believe they may have taken delivery of them with -5C4 engines rather than waiting for CFM to get them out the door.
The A330-200 and -300 are also receiving the same 'X' upgrades except the engine - QF's first 332 was the first "enhanced" 332, and NW's first 333 was the first "enhanced" 333.
There is no A330-200X because the A330-200 was designed from the outset to include the centreline fuel tank, larger vertical stabiliser, enhanced and strengthened wing, and other details that were developed for the A340-300X, and later incorporated into the A330-300X.