How many gigabytes of ram does my motherboard support?

Solution
Revision 4 support.


2 x 1.5V DDR3 DIMM sockets supporting up to 32 GB of system memory
Dual channel memory architecture
Support for DDR3 1866(O.C.)/1600/1066 MHz memory modules

http://www.gigabyte.com/products/product-page.aspx?pid=4000#sp
Gigabyte GA-800gm-D2H:
http://www.gigabyte.com/products/product-page.aspx?pid=3515#ov

1.2 x 1.5V DDR3 DIMM sockets supporting up to 4GB of system memory (Note 1)
2.Dual channel memory architecture
3.Support for DDR3 1666 (OC)/1333/1066 MHz memory modules


In laymen terms you can stick two sticks of 8GB of RAM in with a max MHZ of 1600MHZ.

Also this is how much RAM each Windows 7 can use:
http://acer.custhelp.com/app/answers/detail/a_id/2729/~/what-is-the-maximum-amount-of-ram-memory-supported-by-windows-7%3F

(The 1666 is the max overclock the board allows)