Card Specs Needed

I have just finished building my desktop and I need to replace my 8800 GTS 512 because it isnt keeping up with rest of my components. I do gaming such as the Total War Series, Fallout, and AC3. I also use 3D design programs such as Autocad, Sketchup, and Pro E. I am lost on all the different specs for a graphics card these days and need some help. What does the memory bit mean and how much memory do I need for powering two monitors (One for gaming in HD other for browsing and music)? Also is PCIE 2 really worse then PCIE 3? Or is that just for hardcore gamers. Any card recomendations would also be apprerciated my budget is $200.

Specs:
CPU: i7 3770k
Motherboard: P8Z68-V Gen3
Memory: Geil 16gb
PSU: Antec 620w

Thanks for the help!