First of all I love hardware, if I had the chance to have the best of everything handed to me as it lands I would. Needless to say this isn't a feasible method of upgrade for the majority of PC users. Upon contemplating just going for it for no other reason than because I just can I have come to these conclusions. All of these are a matter of opinion to give help and are not based on any real world benchmarking or research.
Cpu- These have come a long way since their mainstream adaptation for everyday computing, and in more recent years the performance / incurred cost ratio of buying a next gen processor yields a nominal change for the average user / gamer.
Unless you're running severely a severely outdated CPU lets say 5+ years there will be little benefit to upgrading these. They also last FOREVER, There are literally thousands of CPU's at work [ Pentium 4 , Core 2 Duo etc] covered in dust, thrown around (these are LGA (no physical pins) mind you) and who knows what else by the time they get to us (we dont resell anymore) they actually still work. Your CPU will likely outlast your other components as long as you dont keep it pegged at a high temperature.
I have a 4770k [Released Q2 2013/ 2 years old] and I run it stock most of the time (sometimes i play with the clocks), and as much as upgrading seems nice there is no justification at all for it. I've come to terms with the (personal) stupidity of upgrading and will keep this for at least 2 years more. Heck in my opinion if I hadn't had to sell my old 2500k rig I would still be using that for a few years because that generation had one of the best and easiest OC/Temp ratio in recent history IMO.
If you are on DDR2 still, please upgrade. This doesn't get a new generation every year and DDR3 is beginning to be phased out. If you are on DDR3 I say keep it, even higher clocked 2ghz+ ddr3 won't yield much of a difference for any of us normal non heavy rendering/editor users. As for DDR4 it's an expensive jump currently and at least for currently slated processors they will be backward compatible with DDR3. I can't say much more about ram besides these are probably one of the more suspect components if your computer goes down.
So many kinds, these can last a long time too as long as the capacitors are in good shape and there are no power delivery issues or shorts on the board, some are also just a pain in the ass inherently. No real reason to upgrade unless changing CPU's (chipsets are a big deal) or side grade unless you want different options down the road and still want to keep your CPU.
These things have become beastly, and they have to with advances in the programming and engineering that makes recent games look beautiful. With greater demands coming with 4K/future higher resolutions video and gaming even high end cards are pushed to the limits of their capabilities. But for those at lower resolutions and those who don't need bleeding edge power for their video you can probably get away with every other generation if you want to keep up with max settings gaming at its current rate. You can also wait longer and get away with some lowered settings, I believe GPU's to be at a happy medium at the moment to which you can either take it or leave it on upgrades but you will still notice a decent performance improvement by going newer.
Honestly these are the biggest itch for my upgrading habits basically because of what I've said above. I only currently game at 1080p but newer games are always coming out, I'll probably be able to hold onto my 970 for a while but newer (and freaking cool looking) GPU's come out every year. It's a matter of funds vs want vs willpower.
- SSD: Seriously get one, they're becoming more and more affordable and it's worth it.
- HDD: Still excellent for mass storage where SSD cost/space ratio still falls short.
These last a while and can survive a complete system overhaul so use it till it breaks.
Power Supply: When first buying these, DO NOT cheap out. Look at what you need and go with something highly rated that is at least 80+ certified. When upgrading these It is usually with a system overhaul or after a few years of service depending on the load percentage and heat the unit had to endure in its operational life. These degrade in their ability to deliver efficient steady and clean power to your system over time and the severity of this is determined by the internal components of the unit, and its average load percentage. The higher the 80+ rating the better and make sure to check reviews about reliability issues or other problems. Don't let it scare you, your unit isn't just going to die because it's old but even if you don't completely change your system It is a good thing to think about how old this critical component is.
[Headroom] Don't get a supply that is exactly what your system draws under normal loads, give some extra wattage headroom to keep peak efficiency and reduce the speed at which the unit deteriorates over time.
I got my PSU last year when I built this system, with my current hardware unless I want to upgrade to a system that uses more power or for aesthetic reasons I plan to keep this another 1-2 years. I didn't go all out on 80+ rating or anything I got what I determined at the time to be a decent supply that wouldn't kill my system when things got tough.
- Fans: These can last a long time or they can be garbage. High end fans can be on the expensive side considering you usually end up buying multiple but will last longer (hopefully), these can also go between system builds and updates with little issue unless of course you require certain colors. Just make sure you have the airflow you need. [Seriously there are entire articles devoted to this stuff]
- AIO: This can also fall under reliability upgrades. I've never had one fail but AIO's (All in one) liquid cooling loops [IE: corsair H100] can fail or have other issues. Since most are self contained and not meant to be worked on depending on warranty and the issues involved I personally believe that I will keep a similar schedule of replacement that I set for myself with power supplies.
- Custom Loops: My grey area as far as PC building goes as I haven't put the money or effort into this, all I know is keep up with your maintenance and check for leaks/ component failure periodically and that changing systems ususally means re-doing the loop. The guys that want to do this should do a great deal of research into what is required before diving into this.
If you have any questions feel free to post them or PM, also anything up here is up for debate.