The Extreme cards have a faster read/write speed than the Ultra cards. Depending on what kind of photography you do, how fast your camera is, and how fast the card reader on your PC is, this may or may not be a problem.
If you do landscapes, you don't need a fast card. If you do motorsports, you may have a need for the faster cards. If the camera is slow to write to the card, it may be irrelevant how fast the card is as the camera may be slower still.
If it works fine for you, then there's probably no need to get a faster card.
You can still get Ultra cards. Example:
http://www.picstop.co.uk/memory-card...nufacturer=188
Picstop are my preferred supplier of cards as they are most likely to be genuine items. The vast majority of cards on ebay are fakes - slow cards branded as being fast ones.
Edit: snappy beat me to it!