2500K vs 3570K vs 2550K which one to buy

PCIe 3.0 isn't going to be an issue for GPUs for a while...GPUs aren't close to saturating the PCIe 2.0 bandwidth yet, the only thing that does so currently are some PCIe SSDs.

The difference between the 2500K and 2550K is marginal; the 2500K has a built-in GPU, the 2550K doesn't. The 2550K has a slightly higher base clock speed but as both are overclockable, that's pretty irrelevant. Theoretically the 2550K can go a bit further than the 2500K, but it's pretty academic for practical performance gains.

The 3570K is the successor the the 2500K and is a very good CPU, however when overclocked it can get a lot hotter than the 2500K. I believe, but not with 100% accuracy, that this can limit the 3570K to 4.5GHz before small speed gains result in excessive increases in temperature (the 2500K can reach up to 5GHz in some cases, but this depends on your specific CPU...not all CPUs are made equal due to minute differences from chip to chip). The 3570K is also a slightly better performer than the 2500K / 2550K due to it being based on newer technology.

For a gaming rig which you wanted to overclock and not upgrade for a few years, I'd have to say the 2550K really, as they're of a price with the 2500K. That CPU will do you for 4 - 5 years (although you made need a GPU upgrade along the way). The 3570K is newer and slightly better, but with the temp issue IMHO it's not as good for a gaming system. However, it's not a show stopper - just something to be aware of.