Actually, I see more difference on motherboard manufacturers than graphics card manufacturers.
In graphics, all the chips and board circuitry come from Nvidia or AMD. The cards differ primarily in the cooling and factory overclocks.
In motherboards, the chipsets determine the features available like how many X16 slots or overclocking support. Each bios will be different.
Motherboards will differ on size, layout and features.
The voltage capability and management are important if you will be overclocking.
What cpu are you looking at?
Do you own it now?