Unfortunately this is well documented but you, like I, didn't find out until it was too late!
The most RAM that 32 bit Vista can take is 4GB but that total also has to include any other RAM in your system like a graphics card or sound card for example.
I have 4GB installed but Vista only recognises 3GB. I can, however, run Vista ok.
Usually the problem with Vista is the speed of the RAM, the slots it is put in and also that you should have matched pairs.
My mobo is an Asus and I got loads of info from their forum. I should check out Gigabytes forum if it exists.
Unfortunately it won't be easy but I wish you luck!