Districts are identified by letters and/or numbers depending upon where they're referenced. The districts are:
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It depends on what letter, if any, is next to the date. A 1928 C bill from the Cleveland* Federal Reserve District is quite valuable, with prices for a circulated bill in the $250 to $450 range as of 10/2011. Other letters are much more common; please see the Related Question for more. (*) To explain, there's no "Federal Reserve of Ohio". It's actually the Fourth Federal Reserve District which has its offices in Cleveland, so it's generally referred to as the Cleveland Federal Reserve District.
There were no series letters on any 1929 US bills. You may be referring instead to the Federal Reserve District letter; "G" indicates your bill was distributed by the Chicago district. There's more information at the question "What is the value of a 1929 US 100 dollar Federal Reserve Note?"
"G" is the Federal Reserve District letter rather than the series letter; if there is one it's next to the date. The fact that your bill is a Federal Reserve Note means it was printed in 1976 or later and is not rare. If you got it in change it's only worth face value.
Regardless of the district or series letter, values as of 09/2012 are $65-$95 in circulated condition and $350 or more uncirculated.
The green seal indicates that your bill is a Federal Reserve Note. Check to see if there's a small letter next to the date, and look for the name of the Federal Reserve District inside the seal with a large capital letter. Approximate values as of 01/2012 are: No letter, Richmond District: $40 to $65 in average condition, $100 or more uncirculated. No letter, any other district: $22 to $45 / $70 "A", Kansas City District: $45 to $95 / $275 "A", any other district: $22 to $45 / $70 "B", Atlanta District: $4 to $65 / $175 "B", any other district: $22 to $45 / $70 "C", Chicago District: $175 to $500 / $1800 "C", San Francisco District: $350 to $1000 / $3500