Here are your parts
Replacement parts for General Electric GSD640G-01 | AppliancePartsPros.com

The sears parts site has actual parts breakdowns for this unit.

Note where the upper and lower spray arm are.
Start a cycle then open the door to see if both arms have rotated.

Are the holes in the spray arms clean?
I find it is easiest to start a wash so everything is wet and the open the door and pull the gunk out of the holes with a pair of tweezers.

Check how much water is in the unit, usually they fill to just under the heating element.
A low fill will cause poor washing.
Often a low fill will also cause the pump to growl as it pulls in air and cavitates.
If it looks low, let the unit fill and then add a couple quarts of hot water. Just be careful that you do not overfill it. If it now washes OK then low fill is probably the problem.
The most likely cause is the filter screen on the input side of the water valve is partially clogged.
Remove the valve and check/clean the screen.
Do not permanently remove the screen from the valve as it is there to keep crud out of the valve which could hold it open and cause a plug.

Remove the sump cover from the bottom center of the tub and check for debris in it or something wrapped around the chopper. Be carefu;l as there is often broken glass in the tub.

Here is a good site with basic dishwasher repair info.
http://www.applianceaid.com/dishwash.html