You may find that 7 speed shimano cassette in the link slightly too wide to fit on your freehub, plus the spacing between the gears will be 5mm as opposed to the 5.5mm, which is regular 6 speed spacing, according to Sheldon Brown's Freewheel spacing crib sheet. Assuming your system is indexed, your gears may not index very well with the 5mm spacing of a shimano 7 speed cassette. Id reccommend this type of cassette instead of the shimano because, unlike that particular cassette in your link, this one can be succesfully taken apart. http://www.ebay.co.uk/itm/Sunrace-Bicyc ... 3f169d8d9b
Just drill out the rivet heads holding it together, or undo the bolts, whichever it has, and leave off the largest sprocket. You can then swap the spacers from your old 6 speed cassette onto the new one for better indexing. Dont worry about the rivets/bolts that held cassette together originally, as they are for convenience really. Thread each cog and spacer individually onto the freehub splines, lock it up and you should be fine.