Doing my first build:
I have a single graphic card, have a Haswell processor, namely i7-4770. I am not planning to OC and it looks like 550W would suit my needs.
I have seen new power supplies supporting C6/C7 power saving states. My question is, is it worth it to spend more on the psu so it would be Haswell-ready or is it not going to make a difference for me?
C6 / C7 States only apply when the PC is under sleep / hibernation / standby states. So no.
yeah i know that, but this for instance analyzes the impact of enabling them on a 4770k system and seems to observe a noticeable difference:
"We found that with C6/C7 disabled that the system would idle between 23.7 and 24.1 Watts, but with them enabled we were seeing between 20.5 to 21.2 Watts."
wondering how that applies to my system.
It doesn't exactly make such a massive impact on your system. It just lowers power usage and temperatures on the CPU cores. As long as your current PSU is a good branded one, you should be fine with Haswell.