I'm not 100% sure of what you're asking, but I shall answer as best I can.
The references to DDR3 on motherboards is to do with the system RAM they use, not the video RAM (VRAM).
VRAM is stored on the Graphics card and is now usually GDDR5. All Graphics cards are usually of the same dimensions (with the exception of the size of the cooler), so as long as your motherboard has a full-size PCI-e slot and there is space in your case, you should be able to fit any card.
MSI make both AMD and Nvidia Graphics cards. I think the question you are probably trying to ask is should you look at AMD or Nvidia cards. The choice between AMD and Nvidia is one of budget. Nvidia cards are on average more powerful, run cooler, and have a lower power usage than AMD cards, but are also more expensive.
What model is your motherboard, and I'll be able to certify if either of those cards are compatible. Blindly I would advise the MSI GTX970 as it is more powerful than the R9 270x, and I have two that have run perfectly since purchase.
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