Yea I would agree a 670 with 4 gigabytes of ram perhaps to help out with that load of 1440p would help. 2 would help if you wanted more steady settings at 1440p but yea as a starter a 670 would be good from a Nvidia side of things.
2GB is still fine for 2560x1440/2560x1600 as long as AA is kept at 4xAA or lower.
I'm not saying the idea to get 4GB cards is necessarily bad, however to utilize them, dual 670s or 680s are a must.
I can agree with that normally AA+AF is just eye candy anyway it doesn't really make a whole lot of different it seems in games I turn it on most of the time now to avoid tearing honestly for getting frames well beyond what my screen can handle.
what i think is at that resolution would not need AA or maybe just 2x AA. You have to magnify to see the difference. Not real world difference
Well the purpose of AA is really to smooth objects out. So I'm not sure definition really fixes that. Internally the game will use your graphics card to smooth out jagged edges to make items look more smooth more defined. It's not necessary if you want more of that I would get a 2nd card.