QUICK ANSWER:
The Asus 760 4GB Stryker costs MORE than an Asus GTX770 2GB. You're far better off betting the GTX770 which averages 21% faster. There's arguably NO GAMES that benefit yet above 2GB for this performance level (Watch Dogs doesn't count; it's getting an update to fix the memory issue).
MORE INFO:
No point in worrying about future VRAM usage when you are getting lower performance with the GTX760 vs GTX770 in every game right now.
GTX770 vs 760 (770 21% faster on average):
http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_760/27.html
Asus GTX760 4GB Stryker-> $309
Asus GTX770 2GB-> $300 (including Watch Dogs)
http://pcpartpicker.com/part/asus-video-card-gtx770dc2oc2gd5
Hey,
1) GTX760/770 is okay but you will have to carefully tweak some games to keep your frame rate goal. GTX770 will still serve you well for quite a while.
2) 4GB vs 2GB: Yes, I know this has been beat to death but it only makes sense to get a 4GB card if it doesn't cost much more for comparable quality as there's no benefit yet.
3) CPU:
No mention of your CPU. Many games will be bottlenecked by any AMD CPU such as Skyrim which is about 40% with an FX-8350.
If you've got a modern 4-core Intel then you're generally okay such as an i5-2500 or better.
4) POWER SUPPLY:
The GTX770 requires a 6-pin and 8-pin PCIe power connector so use that as a guide. Your CX500 does appear to have the required connectors as it supports up to 2x8-pin I believe.
Summary:
a) The Asus GTX770 2GB will outperform the 4GB version in every game. Don't get the 4GB version.
b) Your PSU appears adequate up to a GTX770.