I have a three rigs I built myself. One has a Rampage III Black Edition motherboard, and as much as I complain about Asus, that motherboard is still great after all these years.
Here is what I did to upgrade it. First I got a new hard drive--an SSD--that made a huge difference. Then I got a new GPU, and then two in SLI (switched from a gtx 580 to a 1070 and then to two 1070s in sli). I have switched out one set of cards for another--both do well in it (switching from two 1070s to two 2080s). That doesn't mean you need to run out and get two gpus, but I did--and the difference was real.
As for building a new system, I built a system with a Rampage VI Extreme Omega (Intel) and just last week another with a Crosshair VIII Formula (AMD). The Rampage VI Extreme Omega was fine for nearly a year, but it crashes now when playing World of Warcraft--and since 8.3 was released under load. But the system is so beefy I still don't get what is going on--but the latest BIOS from Asus also causes the CPU to run overclocked at default settings. As for the Crosshair VIII Formula--building the system was a snap, and the installation of Windows went like a dream--that was not the case with the Rampage VI Extreme Omega. Installing Windows was a nightmare with that system and it had lots of problems with Windows--hence I had to play around with the registry because the system had one of the CPU cores running at 100 percent.
Another thing I did (with the Rampage III Black Edition) was ditch Windows 7. Windows 7 was a nightmare, and some of the updates simply would not jive with the system (the Rampage III Black Edition). Sometimes I had to rollback the updates, other times they simply would not install. Other times there would be errors, etc. And my system too would take forever to start up, and needed to literally be left alone for ten minutes or it would not work properly if you did anything. The upgrade to Windows 10 was easy and the system now works perfectly (but you cannot install any Asus software once you do that--which, ironically is a GREAT thing). Yes, a GREAT thing (seeing as hardware is Asus's strong point--the software, frankly speaking, is often buggy and just plain poor).
As for a CPU upgrade, my Rampage III Black Edition has a 990x--the best you could get a the time (a six core processor) and it, along with the motherboard, this CPU has stood the test of time. One thing though--I would not get an Asus AIO water cooler and would be sure to get one that responds to the CPU core temperatures vs the CPU socket temperatures. It would also be nice if Asus provided users with a way to monitor the coolant temperature like Kraken X62 does. I give the Kraken X62 software (Cam) five stars because it allows you to watch you coolant temperature (which HWMonitor does not). Asus software is often poor and is widely criticized (along with Asus after care) as reflected in the very poor ratings in the Microsoft store that Armory Crate gets. Never mind the regular complaints on this board about RGB management--which I have given up on and pulled hair out trying to get working well.