I will soon buy a new desktop and I'm stuck with 3 different computers
I'll a buy pre built custom pc but there's one thing
The one with i5 2500k costs 800 € and the 3570k costs 850 € (everything else is same)
They are rather close in performance but the 3570K will have technology exclusive to it and paired with a Z77 you will have access to those technologies. The issue comes down to what the 50 Euro's can contribute towards, if it can get you towards a better GPU or PSU then it is very worth it to rather settle with a 2500K or 2400 and put the money towards another component. As for the 2700K, if you are not into rendering or multithreaded apps then you have very little use for a 2700K and will be running a very expensive 2500K.
No. It is not worth an extra 50€ for the i5-3750k.
Just get an i5-2500k and Z77 motherboard so that you have a PCI-e 3.0 slot. I do not believe there are any video cards that would be bottlenecked by a PCI-e 2.0 slot, but you might as well "future proof" yourself.
Most applications (including games) do not make use of Hyper Threading (HT). The only common applications that most people use which can take advantage of HT is file compression/decompression, but how often do you do that? Video encoding and 3D rendering can take advantage of HT as well as financial and scientific modeling. Some PhotoShop processes can take advantage of HT if I am not mistaken. If you don't do any of that, then there's no need to buy an i7-2700k.
Basically the only differences between the 3570k and 2500k are: (1) 100 MHz higher base clock and max turbo (but if you're OCing, irrelevant); (2) very slight (no more than a few percent) performance improvement per clock, but lower max OC, so it's a wash; (3) lower power usage, but on a desktop, barely relevant; (4) better integrated graphics, irrelevant for gamers.
The other technologies that Z77+Ivy Bridge enables, compared to Sandy Bridge+Z68, are pretty trivial for most users, gamers included. PCIe 3.0 provides bandwidth that very few cards need, and some Z68 motherboards already had PCIe 3.0 support anyway. So I don't think it's 50€ of technology. And the prices of Z68+SB will stay low for a little while as sellers get rid of their stock.
There's no harm in going Z77 probably, because the boards are barely more expensive (if at all) than Z68. But I don't see the real reason to go IB right now, not at the prices they're charging. When they cost the same as SB, then of course IB will make sense.
Here are some reasons that you might get the IB version (3570k) instead of the SB version (2500k):
(1) power usage will be lower and will pay off the difference in the long run--could be a while, though, depending on your habits and cost of electricity
(2) if you don't plan on OCing heavily, the 3570k will have a slight performance gain because of slightly better per-clock performance
To me, those aren't worth 30€. They might be worth 10 or 15€. Really, I know it's weird not to get the latest and greatest, but IB has pretty minimal relevance for desktop users and especially for gamers.
Yes, that's the right way of thinking about it--if you don't drop the 30 on the processor, you can get something else! And if you are not otherwise going to get an SSD, or if (e.g.) you'd be stuck with a 64 GB instead of a 128 GB, that's going to be a much better expenditure of your money. Honestly, the 2500k is already overkill for virtually any game right now.