To clear up things:
Motherboards are of many brands, like GIGABYTE, ASUS,INTEL, ASHROCK, DFI, MSI, etc.
All above are built either on an INTEL chip, NVIDIA chip, or VIA chip. (The chip that is responsible for the main functions on the motherboard.)
A motherboard of any brand build on an INTEL chipset support only INTEL proccessors .
Motherboard built with NVIDIA or VIA chip support both AMD and INTEL proccessors.
Any motherboard with a 16x PCI express slot can use any graphic card (NVIDIA or ATI)
If you want to use two (2) graphic cards working together as one(this called SLI mode from NVIDIA, crossfire mode from ATI) then you need a motherboard with two (2) PCI express 16x slots on your motherboard.
Motherboards build with INTEL chip support ATI CARDS in crossfire mode, Motherboards build with NVIDIA CHIP support SLI mode.
Once again if you use ONE graphic card you can use any motherboard with any chip if it has at least one PCI express 16x slot.
You can see if a motherboard support all above futures on motherboard specification quide.
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About memory and Proccessor.
Amount is one think clock speed is another thing.
Amount of memory count on Megabytes (MB) ,CLOCK SPEED COUND ON MHz(MegaHeartz)
With amount of memory we mean how much information a memory chip can handle.
With clock speed we mean how fast an amount of information can be proccessed.
So it depend on how much money you have , what you want to do and how fast.
ASUS, Gigabyte or MSI motherboard are great motherboards, prices varies depent on features a motherboard have,
If mainly cause are the games, Internet, and word proccessing, then a Core 2 dual CPU at 3GHz is enough, as in games most job now done from graphic card.
Quad core CPUs cost more than dual core and in some cases perfome worse inb a game.
Memory an amount of 3GB is enough, and a graphic card is depend of the money you want to spent.
I recomend 9800 GTX from NVIDIA as it performs BETTER OF ANY ATI at a price range between 150-200$ .
Now think what you want to do, the money you have and post back with any other questions.
Have a nice day