question vRam in crossfire/sli

Nl_BugsBunny

Honorable
Oct 8, 2013
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10,510
new generation for games, my question vRam. who can tell-me what is more right. i will go upgrade my graphic card from ENGTX560 Ti 1gb to another more strong and with more vRam. and im noob in crossfire/sli. i have listen games go use a lot of vRam and actually i play BF4 in High with this Asus and my 2700k at 4.5ghz. on my 22'' 720p LG screen. i will upgrade to 24'' full hd this year and next year more two of them. and then a will put more one graphic card in my set.
my question is also know if in sli or crossfire im only get vram from one of them? if i buy one with 2gb, and then in future one with 4/6gb. and then i get with that quantity of vram? or have to be the 2 graphic card's the same memory?
thanks.