Geotransformation is not valid

Hi All,

I'm trying to find the area of my Denver, Colorado polygons in kilometers (km) and I think my data is in Decimal Degrees, so I need to project it before using calculate field to calculate the area.

My data properties say the following:
Data Type: Shapefile Feature Class
Shapefile: C:\foo.shp
Geometry Type: Polygon
Coordinates have Z values: No
Coordinates have measures: No

Geographic Coordinate System: GCS_WGS_1984
Datum: D_WGS_1984
Prime Meridian: Greenwich
Angular Unit: Degree

My specific questions are:

1. I think this means my data is in decimal degrees and I need to project it to get an accurate area measure in km. Does this sound correct?
   
2. I believe an appropriate coordinate system would be:
   NAD_1983_HARN_StatePlane_Colorado_Central_FIPS_0502_Feet
   
   My source of coordinate system selection is from here:
   http://www.denvergov.org/?TabId=426422
   
   http://www.spatialreference.org/ref/?search=Colorado
   Does this sound correct?
   
3. The project tool requires a coordinate system transformation and I selected NAD_1983_To_WGS_1984_5. Does this sound correct?
   I based this selection on this source: http://blogs.esri.com/esri/arcgis/20...the-right-one/
   
   which says:
   HowTo: Determine which NAD_1983_To_WGS_1984 transformation to use--
   NAD_1983_To_WGS_1984_1 – for the entire North American continent.
   NAD_1983_To_WGS_1984_2 – for the Aleutian islands.
   NAD_1983_To_WGS_1984_3 – for Hawai’i.
   NAD_1983_To_WGS_1984_4 – superseded by _5; this transformation method should no longer be used!
   NAD_1983_To_WGS_1984_5 – for the 48 contiguous United States.
   NAD_1983_To_WGS_1984_6 – for the Canadian province of Quebec.
   NAD_1983_To_WGS_1984_7 – for the Canadian province of Saskatchewan.
   NAD_1983_To_WGS_1984_8 – for the Canadian province of Alberta.
4. 
   I get an error when I run the Project (Data Management) tool.
   Here's a picture of how I filled in the Project GUI: http://screencast.com/t/ARKUS3wQdf
   This is the error I get:
   ERROR 000151: The geotransformation(s) is not valid. Failed to execute (Project).
   Any ideas why this may be happening?

Thank you for your help!