say a dual gpu card like a 590:
Its 1 card (3gb total memory), but it has 2 x GPUs on it, so its 1.5gb pr card (1.5gb dedicated memory pr GPU).
"what do you look for in a graphics card"
performance/$ + features + good cooling solution/low noise.
So it starts with you looking at your wallet, how much do you want to use?
then you look up benchmarks for the games you want to play, and buy the card that gives you the best performance/$.