In agreement with nekulturny, The best bangs for the bucks is the video card upgrade.
Also currently agree about i5-2500K and a Z68 MB. For two reasons, A) The IB MBs are too new, as Ive learned the hardway you should always wait and let others DEBug the MBs B) on a clock-clock performance IB just hits the point of being able to tell the diff, and OC head room is lower. Instead of looking at the IB vs SB CPU, look at MB features to determine the best buy based on Cost differencial.
I would agree that if you do upgrade CPU/MB, Intel is a better choice, and it is NOT because of the CPU as Both will perform about equally as well in gaming.
My rational is I prefer the Intel Chipset:
.. Intel seems to come out with Update drivers quicker than AMD and generaly better. Probably because of R&D budget and Manpower. AMD has been hurt in both these areas ( Look at stock evaluations for info on this aspect ).
.. For Hardware and software 3rd party developers look at the Bottom line which is based on market shares. Ideally a 3rd party harware or software should (and in many cases does) perform equally as well on both plateforms. But we live in the non-ideal real world. The developer will most often design their product based on intel performance and let the chips fall where they may.
The two points I bring out are NOT any Fanboy - just cold hard facts.
Either system is going to be about the same if gaming is the only consideration.
But if you do upgrade - PAY close attention to the MB specs with an eye on the Future (upto 3rs).
.. Number of sata II/III slots. Sata III is NOT important for HDD, but is for Newer SSDs.
.. Number of USB 3 ports, And the MB has the 20 Pin header to connect front panel USB ports - There have been a nuber of individual that have been bitten by this.
.. pci-e slots. If you even remotely think that you will end up buying a 2nd GPU for xfire/SLI you want x8/x8 NOT x16/x4!!