In all honesty, people with i7-2700k's clocked to 5ghz and a GTX 680 had crappy frames - myself included though I only have an i5-2500k (but I do have a 680).
So, I would seriously just wait for release to come to make a decision.
One more thing to remember - and perhaps most important because money is a key factor for you - is that with ANY of the 4 core CPU's that you are looking to upgrade to, you will be REQUIRED to buy a new motherboard with it to match the new CPU. This is AT LEAST an extra $80-$100 on top of the cost of the CPU (not to mention compatible RAM depending on the type of RAM you guys have.)
By far, the cheapest option for you to upgrade would be buying this particular CPU:
http://www.amazon.com/Intel-i5-650-P...5973879&sr=1-3. This one will work in the motherboard you both currently have and it is a 4-core cpu.
The reason I say to wait, however, is that you may honestly not get $200 worth of a performance increase with the new CPU upon release. It is a modern game, but that does NOT mean that it will utilize more than 2 cores upon release anyways, so you may very well be wasting your money for all you know. Seriously, its a bit silly to judge a game's performance through a BETA. Hell, the poor performance could be because of many reasons:
1. Game is not optimized for ANYTHING yet.
2. This was as much of a stress test on servers as the earlier announced official stress test. All the lag could be due to the fact that the servers were not ready/capable of handling the load yet. All the computer horsepower in the world isn't going to alleviate lag from this source.
3. Your internet connection was/is crappy during that time and thus introduced lag.
4. Did I mention the game is not optimized for ANYTHING yet?
Considering the minimum specs required for the game, upon release, I think you and your BF are going to be JUST fine playing at 1600x900 resolution.
Keep what you have, if you find that its not good enough when the game is released, decide on your course of action then, using benchmarks from various sites on what is the best thing to upgrade.