Yes, not a problem, but why go to Dura-ace? Ultegra does the job just as well and, being honest, you won 't notice the difference - at all - between 105 and Ultegra, unless you're a road racer or a pro. Keep your money and buy what does the job sensibly for what you need to do.

The following would be suitable.

http://www.wiggle.co.uk/p/cycle/7/Shimano_Ultegra_6700_10_Speed_Cassette/5360042802/

Standard fit ... ? 12-25T ... or what combination? Is it 20 speed or 30 speed? ie: two front chainrings or three? It all depends on where you ride and how you ride. If it's hilly, you need bigger sprockets on the cassette (28-34T) to make it easier to climb in a lower gear with your "granny" ring or smaller front ring (30 or 39T). If you're powerful and it's flat, you need smaller sprockets (11T) on the back working with your big front ring (52/53T). And yes, the gear ratios are important because you can work out what you are pedalling with a little maths. You can read about it here: http://sheldonbrown.com/gears.html

This is a 10 speed cassette, if you have 10 speed Shimano at the moment it will be fine.

Only the bottom smallest ring is a separate item. The rest of the rings are pinned with the top three rings on a alloy carrier.

At this time we do not have an alternative with the ratio range required, its closest is the Ultegra 6700.