The Sauce: here's a Fraps run the other day with my rig 1 below with 2 GTX670s (EVGA GTX670 FTW with stock core bumped by +50 and memory bumped by +400).
Here's some frap data for BF4 runs on Ultra with 2xAA in rig 1 below:
2013-11-01 21:27:50 - bf4
Frames: 860 - Time: 4563ms - Avg: 188.472 - Min: 174 - Max: 199
2013-11-01 21:27:55 - bf4
Frames: 14100 - Time: 146359ms - Avg: 96.338 - Min: 27 - Max: 202
2013-11-01 21:31:36 - bf4
Frames: 479 - Time: 2391ms - Avg: 200.335 - Min: 199 - Max: 201
2013-11-01 21:31:41 - bf4
Frames: 4092 - Time: 36110ms - Avg: 113.320 - Min: 13 - Max: 201
2013-11-01 21:32:43 - bf4
Frames: 4335 - Time: 76578ms - Avg: 56.609 - Min: 25 - Max: 196
2013-11-01 21:34:01 - bf4
Frames: 3296 - Time: 36671ms - Avg: 89.880 - Min: 74 - Max: 113
2013-11-01 21:34:38 - bf4
Frames: 2394 - Time: 32282ms - Avg: 74.159 - Min: 17 - Max: 102
2013-11-01 21:35:12 - bf4
Frames: 6241 - Time: 90656ms - Avg: 68.843 - Min: 25 - Max: 96
2013-11-01 21:36:44 - bf4
Frames: 6954 - Time: 104140ms - Avg: 66.775 - Min: 3 - Max: 94
2013-11-01 21:38:29 - bf4
Frames: 64684 - Time: 865281ms - Avg: 74.755 - Min: 16 - Max: 107
2013-11-01 21:52:55 - bf4
Frames: 7388 - Time: 121516ms - Avg: 60.799 - Min: 0 - Max: 121
Hope this helps.
Since you already have a GTX 670 adding a second WILL improve fps significantly. Likewise you ARE limited to 2 GB Vram so you have a decision to make.
BTW: Just for giggles, before I left for work this morning I ran 3D Mark Firestrike, both regular AND Extreme for rig 1 and here are my results with the latest approved non-beta drivers:
Regular
Overall 11,644
Graphics 14199
Physics 15600
Combined 4266
Extreme
Overall 6113
Graphics 6616
Physics 15647
Combined 2462