Short answer:
1) I personally would wait for a high-end Maxwell like a GTX880 4GB or whatever, or at least a little longer to find out release dates.
2) If not, my advice is to get a GTX780 3GB (ie EVGA 967MHz model for about $500 USD).
3) The GTX670 prices are way overpriced too due to supply/demand. This should assist in SELLING your card used. So, perhaps research pcpartpicker new and E-Bay used prices and sell the card, take the money and put it towards a GTX780.
LONG ANSWER:
First of all, don't play at 2560x1600. Choose 1920x1200 instead.
If you could maintain 60FPS with almost MAX quality settings that would be different, but there are better things you can do with the processing power.
I'm not certain if the game actually uses more than 2GB of VRAM even at 2560x1600. If not, it's a waste to upgrade and a GTX670 to a GTX770 isn't worth the money IMO.
In that case, the GTX780 3GB would likely be a better option then sell your current 670.
The proper method to TEST how much video memory is used is rarely done (can't with Windows 8 AFAIK). It's done like this with W7 or previous:
a) Disable AERO
b) reboot
c) Run Process Explorer:
http://technet.microsoft.com/en-ca/sysinternals/bb89665...
d) "View"-> "System information" -> "GPU" -> "GPU dedicated memory"
e) It should be pretty LOW (preferably under 50MB)
Then start the game and play for up to 15 minutes, close or alt-tab to view the history. As long as it's not hitting close to 100% getting more VRAM won't matter.
Why is it done incorrectly?
Most people just monitor VRAM usage without reducing the usage in Windows to a relatively small amount. Windows can MOVE the Windows video memory if needed. For example, in BF4 people tested the game on a 3GB card on a high-res monitor and discovered it used 2.2GB.
So the game uses more 2GB was the conclusion. Make sense?
However, with 2GB cards only 1.8GB was used. What happened was there was roughly 400MB already for the fancy Aero or whatever data. In the 3GB case Windows had no reason to move the data. In the 2GB case it did. Once VRAM usage got high it moved the data to the System RAM (i.e. DDR3) so that roughly 1750MB was used by the game. Once the game closes it's quickly copied back as well.
GTX770 4GB vs GTX670 2GB:
If the extra memory doesn't matter (which I doubt) there is only a 13% maximum difference if comparing similar frequency/quality of cards like Asus vs Asus.
That likely is at most 10% average though your CPU factors in. Put another way, to go from 50FPS to 55FPS would cost close to $400 USD for a GTX770 4GB (minus whatever the 670 sells for used). Ridiculous, and don't consider the cheaper Zotac/Powercolor 4GB models. Lower quality and lower frequency would likely mean closer to NO benefit at all.
SUMMARY:
I really think the best way to go is
- drop the resolution to 1920x1200,
- scale back the quality settings to achieve your desired frame rate.
- Use Adaptive VSYNC for this game (not globally) and tweak to maintain 60FPS at least 90% of the time, OR optionally disable VSYNC and tweak to 40FPS average roughly (I'd say use GEFORCE EXPERIENCE but I disagree with it most of the time since it tends to use the max resolution regardless)
- Then, finally, wait and get a high-end MAXWELL card once released. I'm guessing three to six months.
(WATCH DOGS should run pretty nicely in the mean-time if tweaked properly. It will also get better as it has several bugs and optimization issues that are being worked on by the game dev, and almost certainly some NVidia enhancements as they've been doing a lot of DX11 enhancements for various games. So 5 to 10% boost by NVidia alone is possible.)