Shimano Cassettes - Sheldon Brown: http://sheldonbrown.com/k7.html
7-speed Cassettes; 8-speed Cassettes; 9-speed Cassettes; 10-speed Cassettes
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7 Speed: http://sheldonbrown.com/k7.html
http://sheldonbrown.com/harris/k7.html#7cassettes
13-30T: 13 15 17 20 23 26 30
14-32T: 14 16 18 21 24 28 32
13-34T: 13 15 17 20 24 29 34
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8 Speed: http://sheldonbrown.com/k8.shtml: 11-32; 11-34
11-30T: 11 13 15 17 20 23 26 30
11-32T: 11 13 15 18 21 24 28 32
11-34T: 11 13 15 17 20 23 26 34
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9 Speed: http://sheldonbrown.com/k9.shtml: 11-32; 11-34
11-32T: 11 12 14 16 18 21 24 28 32
11-34T: 11 13 15 17 20 23 26 30 34
11-36T: 12 14 16 18 21 24 28 32 36
Sheldon Brown custom 9 speed Cyclotourist cassettes
13-34T: 13 15 17 19 21 24 27 30 34
14-34T: 14 15 17 19 21 24 27 30 34
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10 Speed: http://sheldonbrown.com/k10.shtml: 11-32; 11-34; 11-36
12-30T: 12 13 14 15 17 19 21 24 27 30 (Tiagra 10 4600 Series)
11-36T: 11 13 15 17 19 21 24 28 32 36
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Certainly, most bicycles with 7-speed cassettes can be upgraded to 8, 9 or 10 speeds. There'sinformation on this site on how to do that. But, you also can stay with 7 and you don't have to regret it. Some kinds of 7-indexing speed shifters are getting hard to find, but 8-speed shifters will work with a 7-speed cassette and a bit of alternate cable routing.
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GEAR INCHES - LOW END
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GEAR INCHES - HIGH END
50/44/28 X 13-30 = 25.1 – 104 gear inches
50/44/28 X 11-32 = 23.8 - 122.9 gear inches
48/36/26 X 11-32 = 21.9 - 117.8 gear inches
48/36/26 X 11-34 = 20.5 - 117.8 gear inches
48/36/26 X 12-36 = 19.5 - 108 gear inches