I do not think the 2500 will be a particular problem.
I also have a P8P67-M PRO motherboard.
One thing you need to be aware of is the cooling problem with two high powered cards installed together.
The top card will get hot.
If you are using only one monitor, I suspect that a single GTX670 will do the job.
I think I would try that first and see how you do.
You can test how the cpu is doing by reducing it's output, perhaps to 70% and seeing if there is a significant drop in fps.
If your fps is sensitive to this, then add more graphics power may not help much.
On the flip side, reduce your resolution and eye candy. If your fps goes up significantly, then your cpu should be able to handle a second or stronger card.
My thought would be to stick with a good single card, either a GTX680 or 7970 for now.
Next year, when the GTX7xx and AMD 8xxx series launches, you can sell the current card and replace it with the next best thing.