"Good" condition is actually not very 'good' at all. In coin-speak a "Good" coin (capital G) is very worn, with the date, mint mark and denomination visible but most details worn off.
You can check a book such as Photograde to see examples of the various conditions with names such as Poor, Good, Fine, Very Fine, AU, and so on.
An 1879 S Morgan dollar in MS(Mint State)63 condition is worth somewhere around $64. This is a common date.
An 1879 "Silver Dollar" is a Morgan dollar.
1879 S is not a rare date for Morgan dollars. On average if they are in circulated condition they are worth $16.50.
Assuming no mintmarks, circulated Morgan dollars from 1879 are worth $17.00-$26.00 depending on condition.
A U.S. silver dollar dated 1879 is a Morgan dollar. Assuming it is circulated and does not have a mintmark, values run from $25.00 to $40.00 depending on the condition of the coin.
1879 would make it a "Morgan" dollar.
The 1879 Morgan dollar has a diameter of 38.1mm, thickness is 2.4mm.
This link shows a 1879 Morgan Dollar: http://www.coinfacts.com/silver_dollars/morgan_dollars/1879_morgan_dollars/1879_morgan_dollar_obv.jpg
Peace dollars were made from 1921 to 1935. An 1879 silver dollar is called a Morgan dollar after its designer George T. Morgan. There's more information at the Related Question
The 1879 Morgan dollar is not scarce or rare, assuming it's circulated and has no mintmark, average retail values are $30.00 to $41.00 depending on how much wear the coin has.
The 1879 Morgan dollar is not scarce or rare, assuming it's circulated and has no mintmark, average retail values are $26.00 to $32.00 depending on how much wear the coin has.
All 1879 issues have 7 tail feathers.