The longer you wait, the more likely you are to purchase better (albeit minor or significant) components in the future; as new "technology" is always being developed and released. Hence if you can wait for Skylake and are satisfied with your existing setup, then you could wait it out since that will also be the arrival of DDR4 RAM.
With all that said however, on these forums we usually recommend purchasing sooner rather than later; something new will always be released. When Skylake is released towards the end of 2015/beginning of 2016, you may ask yourself the same question: should I wait for Cannonlake? If you want to upgrade you can purchase a H97/Z97 motherboard to support existing Haswell/Devil's Canyon processors, in addition to supporting Broadwell once they're released too. Skylake/Cannonlake will unfortunately require a different socket once again (LGA 1151), hence you will require a new motherboard and in most cases DDR4 RAM to utilise Skylake. I say "most cases" to DDR4 since certain boards will still support DDR3 in conjunction with Skylake; some boards may support DDR3 only, DDR4 only or even both (but 1GB DDR3 + 1GB DDR4 will NOT equal to 2GB, it'll still be 1GB - similar to how DDR2 and DDR3 boards worked).
If you are to purchase a Haswell/Broadwell motherboard and CPU, the processor will still be solid in terms of performance. You should still be able to reach most game's potential until game developers begin to utilise the newer processor's technology and DDR4; for example 2nd/3rd generation Intel processors are still viable and similar in performance to 1150 counterparts (albeit we're only on 4th generation at the moment, we are still awaiting for Broadwell's differences).
You are free to upgrade when you desire. You can upgrade now or even when Broadwell is released, there is no loss in upgrading now if you weigh in the pros/cons. If you are willing to wait, you could until Skylake and DDR4 however, DDR4 is expected to be quite heavy on costs, hence in the ideal situation it maybe better to wait for DDR4 to mature prior to jumping on it as soon as Skylake.
If you make the switch now to Haswell/Devil's Canyon with a H97/Z97 motherboard, you'll be able to upgrade to a Broadwell processor once they're released without switching motherboards. If you purchase any of the 8 series motherboards, such as (but not limited to) the H81, you'll be able to support Haswell processors but not Broadwell, and in certain scenarios you won't be able to support Devil's Canyon either. Regardless with all that said, if you make the switch now to an i5 (heck even an i3) Haswell/Devil's Canyon CPU, you'll still get a significant performance boost over your existing AMD Phenom II X4 965.
It's completely your choice to upgrade now or not.
All the best.
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