I've always been a DIY builder for my machines, and it's been a while. I'm looking to upgrade to an i7 system but I notice now that the retail units available are pretty powerful for the price, and substantially cheaper with the odd OEM concession to parts.
 
Here's a system I whipped up on PCPartPicker
 
And here's a system I scoped out by just hopping over to Dell's site (for example). The biggest difference is the video card, but the Dell system also has lots of doo-dads to round that out.
 
I'm aware a consumer machine won't be as future proof as a custom build, but there's a solid $150 difference in price in the 2, and probably more if I actually hunt around for a consumer box. So, I humbly submit, is horsepower advanced enough that an off-the-shelf machine can function just fine as a DAW?

Both would do. A commercial PC, whether Dell or another brand, will require some work to remove unwanted programs.  However, be advised that Wi-Fi is a KNOWN cause of audio pops/clicks/dropouts (DPC errors).  If you want a PC with Wi-Fi connection, always disable that when recording.
 
I work several PC help forums and the Wi-Fi, as noted, is always the first thing we mention when someone posts with those types of audio problems.

If you're wanting a high-performance machine... a custom build allows you to hand pick each component.
 

you don't want a 'factory' sound card along the lines of a soundblaster...
 
and you don't want video on the motherboard...
 
you don't need ANY software on your machine, except O/s, recording programs.
in the time it will take you to strip your machine from a factory provided OEM software drowning,
you could have just upgraded a custom machine.
 

Noted and agreed on: wi-fi (I only enable it to check for updates now and again), on-board anything (the Dell box has a dedicated GPU, it's not just as well-spec'd as a custom build), annnnd also on a custom box being a better performer. I reckon my question is: is there enough difference in performance at this level to justify a higher-cost custom build?
 
Oh, also noted on default install. I would just assume a consumer machine would need to be wiped and imaged or re-installed with non-bloatware.
 
 

FWIW, That's a question that only you can answer for yourself.   
A custom machine is exactly what *you* want (nothing more... nothing less).
It's built for an exact purpose.
A general-purpose machine is just that...

Haha, yeah but that's only a question I can answer after purchasing one or the other. 
 
Normally I've always been a builder of my DAW, just frankensteining it up with a new CPU/Mobo/RAM once in a while. But not that CPUs are so powerful I wonder if a plain jane machine can do the job and cost way less. Granted, you'd probably run into a part (the GPU looks like a familiar culprit) that gets outdated faster, but if you can get 4yrs or so with it still being up to snuff that's a cheaper investment.

Oh, FWIW I'm hoping to upgrade/replace my DAW for about $1K or less. Since I now need a new: CPU/Mobo/RAM, PSU, and my old case is a POS so I'm thinking a new one, aside from drives (curious about an SSD for the OS) and an optical drive that's pretty much a whole new machine.
 

I would recommend having a machine custom built specifically for audio.
Talk to Jim Roseberry, several forum members have highly recommended the work he does.
OEM computers will work, but they will not be optimized for audio.
I bought my machine from Sweetwater 6 years ago, never had an issue, and couldn't be happier.
That was before I started hanging around here, and learned of Jim's boxes. Jim will definitely be building my next one!
 

i've always built mine after my Gateway 486.  Some OTS machines won't allow modifications if needed. I try to use parts from the old machine to save money.  But replacing the mobo will usually require a new OS, which may also affect license issues w software.  Look into soundproofing and quiet fans or water cooling for a DAW build.