The answer will vary because the criteria are not fixed.
http://www.techpowerup.com/reviews/ASUS/GeForce_GTX_670_Direct_Cu_II/18.html
http://www.techpowerup.com/reviews/ASUS/GeForce_GTX_670_Direct_Cu_II/15.html
The Asus GTX 670 gets 36.5 fps in metro 2033 at your resolution. It gets 264.4 in Dirt 3. The 670 will likely have little problem with Dirt 4 but a bit of a problem with the next Metro.
Your upgrade options include:
OC your 670.... despite popular belief, all 670's are not created equal and you can not generally overclock a reference card to the same level as most factory overclocked cards. This is because:
1. In general, factory OC'd cards come with oversized, improved and more efficient coolers. EVGA, Gigabyte, Asus, MSI generally all provide such on their non reference cards.
http://www.techpowerup.com/reviews/ASUS/GeForce_GTX_670_Direct_Cu_II/33.html
The ASUS DC II cooler is a revelation on the GTX 670, it provides incredibly low noise levels. In both idle and load the card emits almost no noise, it is actually quieter under full load than most other cards on the market are at idle. Not only noise is low but temperatures are great, too. With only 74°C under load the card is cooler than most other GTX 670 cards tested today.
2. Sometimes, not always, manufacturers included beefed up, more accurate voltage control by increasing the number of phase sin the VRM and using improved control voltage control chips. Asus is a good example of of what gets done here with their DCII Cu TOP lines....
ASUS has done away with NVIDIA's cheap voltage regulation circuitry and put a much better CHiL controller on the card, which also supports voltage control and monitoring.
EVGA historically has used reference VRM's on their factory overclocked cards except for their "Classified" line. MSI also does, some reference, some with a minor improvement (i.e Twin Frozr) and Asus comparable or better improvements on their Lightning and Hawk lines.
3. Those that do the voltage control and capacity improvements typically provide modified PCB's with extra beefed up features such is backplates, additional heat sinks and the like.
Obviously, given the above difference, the overclock you can achieve will vary depending on the differences described above.
Buy a 2nd 670 - Scaling on the 670 w/ early drivers was about 68% "on average" with the more demanding titles approaching 100%. This will likely improve so,ewhat w/ better drivers over time.