I'd be replacing the shift cables on the bike before updating the gear.

http://sheldonbrown.com/autoshift.html

http://sheldonbrown.com/derailer-adjustment.html

How many miles do you have on your bike? The cassette may be worn out and need replacing. Worn gears will not hold the chain. Also, have you checked the chain for wear? If it has stretched 1/16 of an inch, it should be replaced. If it has stretched to 1/8 of an inch or greater, it has probably worn your sprockets, too. I'm guessing you are having trouble shifting or the chain won't stay where it should be.

http://sheldonbrown.com/chains.html



Deore is very good; I'm not familiar with the other levels.