GDDR5 is much faster VRAM than GDDR3. At that resolution and performance level, don't waste you money on going over 1GB of VRAM unless it happens to come with a card that is a great price. You'll never use the full 1GB. Another spec to pay attention to is memory bus width. 64 bit, 128 bit, 256 bit, etc. The higher the number the faster the memory will interface with the card's processor.
Just be sure the place you buy from accepts returns in case the old MB doesn't recognize the card.