I wouldn't advise to upgrade to Win10 just yet. I would suggest a clean Win7 install though, but that's just me. I like having a "fresh" system.
I just "upgraded" my system from an old i5 750 and that's just because my son's PC went kaput so I took that opportunity to upgrade my hardware. Having said that, if my son's PC hadn't given up yet, I felt like I could've waited for another year or two. Point is, unless you see a dire need to upgrade your CPU, your 2500K should still be able to keep up with today's titles.
As for GPU, yeah, get the best GPU you can afford. Depending on what PSU you have, so long as it's a good brand and not some cheap Chinese knock-off, a 650W should be more than enough for a single-GPU setup.
I'm currently running a i5 6600K overclocked to 4.5GHz using a Noctua NH-D15 cooler which impressed me greatly. I used a CoolerMaster Hyper 212+ Evo on my old i5 750 so the Noctua was a beast compared to the Hyper 212. Based on what I've read, the Noctua is the best air cooler you can get which trumps all single-radiator AIO watercoolers and even dual-radiators have to work hard to compete with the Noctua. I was so impressed with the product that I also ordered 4 Noctua fans for my CPU cases. I've not really been one for silent-PC builds but since my PSU and GPU fans turn off when the system is not under load, having a "quiet PC" while browsing or doing work is a very cool experience. Sure, the fans whine up once the system is under load but by that time, I'm too busy gaming to notice
One suggestion for hardware -- have you thought of getting an SSD for your OS? 250GB SSDs range around £40-55 and for what they bring to the system, they're definitely worth it!
Hope that helps!