It would be wise to any thinking about upgrading their GPU for this game to hold off.
This game is heavily CPU bound...your CPU will hold you back way before your GPU does.
Now granted if your running some old 9800GT or something, then yeah probably upgrade...but if your running an Nvidia GTX 600 Series, or an AMD HD 7000 series type of GPU, i'd wait...
This game doesn't have a true "Full Screen Mode" it has a Borderless Window Mode...what this means is in Windows, The Game has to send requests to The Window Manager (Windows Explorer and WDM) and it then decides where the pixels are place on the screen, etc....this is also done on the CPU. This makes the game perform like garbage. (This is just very rough quick explanation of how this works)
Give the game a "True Full Screen Mode" would result in about a 10-15 FPS increase for everyone across the board and a much smoother experience....not to mention True Full Screen would allow AMD CrossfireX to work and would also iron out the performance issues for Nvidia SLI as an added bonus...you can use SLI in Window mode, but it scales like poo, and really just doesn't perform well...Nvidia reccomends SLI be run in Full Screen Mode, and AMD don't even bother supporting Windowed Mode for CrossfireX
Bottom line is, right now this game is poorly optimized...CPU is going to hold you back big time no matter what GPU you have due to poor optimization and borderless window.
I am hoping ZOS has Full Screen Mode ready in time for launch...that will make this game run better for everyone regardless of if they are using Single Card, SLI, Crossfire...it will remove a good bit of the CPU overhead as well.
Just so you know, There are folks with Intel Core I7-4770K CPU's (a 340 dollar CPU one of Intel's high End Haswell Chips) that can't keep 60FPS in PVP in Cyrodiil with high end GPU's....the game is horribly optimized right now plan and simple, and getting a better GPU won't change the fact this game is more CPU bound then anything....
the best upgrade one could do for this game is to get I5-3570k and a good Watercooler, and Overclock that 3570k to about 4.2 - 4.4 Ghz to help alleviate the CPU bound nature of this game...I think many will find even after upgrading their GPU, that their CPU will still hold them back significalty....just my 2 cents.