Don't worry about the self build zealots. If you can afford it, why not make it easy on yourself. By the time you add the retail version of windows and other accesories, a mass produced complete package solution isn't even that more expensive, those go on sale too. Plus you get warrenty on the system working, not just the parts. Also just installing everything sounds a lot less work than it is.

I've had pcs made to order in specialized shops, got them from Dell online, altered them myself and bought the one I currently use from Future shop (Best buy eq). That has been my most reliable PC yet... I have kids now, even replacing a battery is difficult with those around :)

Ofcourse if you plan on upgrading you might want to keep some things in mind, like enough free slots for a dual slot GPU and a powerful enough PSU. Or you could just wait until low power more compact GPUs come back into fashion, as they are now.

I think this Desktop PC's going away is brought up every year, and the answer is the same... it might be declining but we're far from them going away. You have too many type of people that need a Desktop...

Gamers, and Steam will help with that in the next year or two...
HTPC's, sure there are small ones like AppleTV/Roku, but lots of people prefer Desktop PC's since they do so much more
Home office, sure a laptop/tablet could do a lot, but you still have people that need a Desktop PC (cheaper/faster/convenient)
Professionals, music/photo/video editing (I'm sure there's a number that prefer laptops for editing on the go), but you have lots who need the power and functionality of a PC (usb ports, graphics cards, hard drives, etc)

That's just 4 types off the top of my head... I'm sure there's lots more. I don't see Desktop PC's going away in the next 5-10 years.

I think a possible future could be your cellphone powers everything, with docks.
You have a laptop with a cellphone dock (no hardware inside), just a screen/keyboard. When you put your cellphone in the laptop, it's now a laptop.
You have a tablet with a cellphone dock (again, no hardware just screen), so when you put your phone in, it's now a tablet
You have a small desktop with a cellphone dock (no hardware), just usb ports, hdmi/dvi/thunderbolt, maybe room for some HDD's, then you have a keyboard/mouse nearby... so when you put your phone into PC Dock (size of AppleTV), bam it's now a computer and you have monitors hooked up to the dock, etc...
You have a TV with a little Dock, plug your phone into that and bam you have a media center

ASUS did a very small version of this with their PadFone, and it was pretty successful in Tawain (I own one), I absolutely love it. Sadly it became outdated, buggy, and and it was pretty large.

The biggest benefit of self build is not price, it is quality and configuration of the parts inside. the mass produced ones nearly always take a ton of shortcuts and cheap options on the internals, fans, heat sinks, drives, memory, airflow, slots etc etc. buying a mass produced dell or HP will give you a solid reliable machine and if you aren't technical at all it probably is the best option and gives you a decent machine. but if you understand the internals and are willing to spend the extra time to buy parts and build you can build a superior machine for a similar price.

Hardcore PC gamers won't buy pre-built PCs. Casual PC gamers, like hardcore ones, are actually growing, despite PC market shrinking. This apparent paradox is explained by the fact that for business and non gaming home computing, and for very casual gaming too, PCs have been powerful enough for some years to reach saturation and need replacements after a longer period of productive use than in the past. Add the facts that PC gamers are a subset of PC users, and that there aren't separated generations in gaming PCs, and the result is that PC gaming can still grow even when PC market overall is shrinking. This doesn't rule out the possibility that x86 PC market be in future marginalised by the growth of a new market spawned by non-x86 tablets. But this new market will replace old PC in all its useful features and functions, while being nimbler and cheaper thanks to having got rid of Wintel legacy.

People always talking about PC sales falling.Fact is You don't need to upgrade anywhere near as much as back in the 90s.My PC i self built just over 4 years ago and still runs many new releases, a PC bought in the mid 90s wouldn't be able to run games that came out two years later.Plus back then PCs were barely fast enough for web browsing or listening to mp3s and you noticed a big improvement in those areas when upgrading, whereas any computer nowdays it's a breeze.

if i recap all previous posts the global desktop market is shrinking, but the pc gaming community is still the same size, only the people who played the sims fucked off

BTW if it isn't too old, most times a PC can simply be upgraded instead of completely replaced. Since my first PC, I bought my first one preassembled, choosing most of its components, then I built myself just two others from scratch, but making multiple intermediate upgrades for each (except the first, that had a desktop case too crammed and with an outdated rear panel).

Just recently switched out my old style HDD for an SSD, made a big difference to performance