[citation][nom]Cryio[/nom]I'm still wondering wether to hold on to my 3.5/4 Ghz Intel Q9550 or to get a non-K 2500/3570/4570.Is it really worth it? Like, how much would be the improvements in applications and games in percentages?[/citation]
I am really beginning to hate these questions. Do some research! Tom's has some interesting (though annoying to navigate) charts where you can make comparisons. Anandtech.com has some less detailed but much more user friendly benchmark options, and there are 100 other sites that do similar things. Also, these are not final builds for drivers, and possibly not even a final build on hardware, so the final performance will probably vary a little bit.
Also 'is it worth it' is a really odd question. There are so many factors there that it is hardly worth answering. It all depends on what you are doing with your computer, what your performance expectations are, and what you are willing to pay to meet those expectations. For my wifes PC we recently 'upgraded' from a Cre2Duo to an i3 computer for her. It was not much of an upgrade as the actual performance is roughly the same for what she is doing (because her performance is mostly bottlenecked by the internet connection), but it was a huge power and noise upgrade. Moving from an average usage power load of ~250W down to ~70W, and from a hot and noisy system to a dead silent system, and from a system that collected lots of dust (due to lots of air movement) to a system that I don't have to clean out every few months, all made that a worthy investment for us to spend ~$200 for the upgrade. That was worth it to us simply from a convenience and power standpoint even though there are minimal performance increases (and actually a decrease on GPU performance), but some people would not think that worth it.
I did a major upgrade to a Sandy Bridge processor ~2 years ago. It was worth it to me to spend ~$800 in hardware to move from my old C2Duo to a SB i7 and a decent GPU so that I could do HD video editing and play high end PC games again. The video editing business paid for the hardware, and helped me take a few video editing gigs that I would not have been able to do on the C2Duo system, so that was worth it. If I did video editing full time rather than 1-3 projects a year then I would probably upgrade to a much larger system, but because that is not my full time job I got something that was right on the boarder of overkill for what I do, but without paying an arm and a leg for truly high end equipment. It is all up to you and your needs vs value system.
You know what SB and IB are capable of compared to a C2Q, and while Haswell is a larger improvement over IB than I was expecting, it is not a HUGE improvement by any stretch of the imagination, so if SB or IB was not worth upgrading to, then most likely Haswell is not going to change that at all. More likely than not your system would see more noticeable performance gains by moving up to a SSD system drive than moving to a more modern processor. But at the same time, if your system was stolen, or died then a haswell system would be a nice upgrade to have.
Lets look at it another way: Intel is not competing with AMD any longer. Intel is competing with ARM and nVidia, while still trying to keep it's large corporate server customers happy. So we are seeing some CPU enhancements, but most of the focus is getting the TDP down, and iGPU performance up to par. Somewhere in the Haswell/Broadwell range of chips I think Intel will have finally caught up with where they want to be on the TDP and iGPU front, and then we will see a small return to performance and more interesting architecture improvements with the Skylake/Skymont series of chips. But then again, if there is no longer any major competition, and if chips are beginning to be small enough to be ubiquitous then there may not be any major reason for Intel to push the performance envelope any longer other than to compete with themselves in getting people to upgrade their old Intel chips.