Originally Posted by Jeff Wills

As others have said, there is no "should"... it's a matter of what you need or want.

Shimano compatible 8-speed cassettes are not as common as 9-speed, but they are still out there. They are generally cross-compatible among brands (except Campy). The limiting factor is whether you want to change your rear derailleur: your existing derailleur can probably shift to a 28-tooth large cog, but you probably need a "mountain" rear derailleur if you want to use a larger (30 to 34-tooth) big cog. Changing the rear derailleur will not require a different shift lever.

A 53-11 high gear is quite high and generally useless unless you live in an area with extended downhills. That's the high gear I have, but I only get to use it on a continuous 3 to 5 percent downgrade. (It is kind of fun to pedal past other riders who are spun out and coasting.)

Some possible sources of 8-speed cassettes:
http://harriscyclery.net/product-lis...-8-speed-1243/
http://www.interlocracing.com/cassettes_steel.html
http://www.interlocracing.com/cassbreakdown.html
http://www.biketoolsetc.com/index.cg...settes-8-Speed

Note that you will almost certainly need a longer chain if you install a larger cog. Replacing the chain when you install a new cassette is a good idea in any case.