Well I've tried both Off the shelf and purpose built - if you are a reasonably heavy user there is no doubt in my mind, a purpose built system from someone who knows what they are doing will ALWAYS be the better option.
I have a couple from Jim - best machines I've ever had and worth every penny - oh and both still going strong!!
Nigel
 

@Bajan - Jim, above, built them for you?

Jim Roseberry
BTW, If you have the right machine, there's no need to turn off those Convolution reverbs in Kontakt/etc.
A modern well-configured machine can handle it... even at small ASIO buffer sizes.

You can't globally turn off convolution reverb unfortunately,and this statement is entirely missing the point.

Having about 20 Kontakt presets running the same number of convolution reverbs is a crazy waste of CPU. Not only that the convolution reverbs in Kontakt are extremely inefficient. It is quite a common experience even in high power PCs.

If you must use convolution reverb you can send them all down to a single effect via the aux bus in Kontakt 5 or to perhaps a far more efficient reverb in Sonar.

... Or alternatively use a sledgehammer to crack a nut.

Jim Roseberry
When you're comparing an off-the-shelf machine to one where you hand pick literally each component for maximum DAW performance and minimum noise, there's absolutely no way any off-the-shelf machine is equal... let alone superior (not even a Mac Pro).  That's ignoring the obvious fact that off-the-shelf machines are built for general-purpose use and to a specific price point.  

To imply though that an off the shelf PC can't run a DAW perfectly well is utter bull. In most cases the specifications far exceed Sonar's minimum requirements, probably the most important component is the audio interface. Of course one is assuming the user will be buying a PC with at least half decent specs.

Most people using DAW's don't need to play counterstrike at the same time...

To the OP, if he's new to PC's and spec'ing them out I strongly recommend posting here for advice:
http://www.tomshardware.co.uk/forum/

It's also has an extremely useful website if you wish to study reviews and the ins and outs of PC specifications.

I think HP Z workstations are pretty good myself but not sure about budget... Best ask over there anyway for more impartial advice.

@Bajan - Jim, above, built them for you?
 
Hi Joshua
Yes Jim above - all our music and videos are made on Studio Cat Machines - Sonar for Music, Vegas for videos
His website is:-
http://www.studiocat.com/open_cart/
Cheers
Nigel
 

To the OP, have you tried running DPC Latency Checker ? If it's indeed related to WIFI or HDD rpm issue, your existing PC may still run Sonar fine after settling it.

Doc, we've been around and around on this.  
 
For a very niche/specific application, it makes good sense (if you can afford it) to build a machine specific to that particular task.  You take away variables from a "general-purpose" scenario... that would result in compromises for a DAW (noise, lower than maximum possible performance, etc).
 
Again, if you can afford it (not that much more expensive), why live with significant compromises?
In the case of running Kontakt, yes... you could go in and disable Convolution reverbs on every instrument.
You can also freeze all virtual instruments... and all EFX/processing to all audio tracks.
That type of project would run fine on a 500MHz machine.  These are compromises we made 10 years ago.
Lucky for us, with today's speed/technology, we don't have to live with those compromises.
With the right machine, you can run these types of projects in realtime... at low ASIO buffer sizes.
We've got numerous hard-core composers taking this example to the extreme (off the top of my head - Sean Beason, Mike Worth, Evan Jolly, and Joseph Briggs).  Sean is probably the most extreme composer I've encountered.  He's regularly pulling 2000+ simultaneous notes of disk-streaming polyphony.  
Sean is doing this in real-time under tight deadlines.  There's no time for major workarounds or other impediments that kill work-flow.  There's no way any off-the-shelf machine would meet Sean's needs.
While most end-users don't need a machine like Sean's, they benefit from not having to implement work-arounds.
When you've got a great idea in your head... and you're ready to lay down that take, do you really want to switch focus to turning off convolution reverbs, freezing tracks, etc?  
It doesn't cost that much more to have a zero compromise machine.
Over the life of the machine... the cost difference is nothing.
 
Regarding the HP Z series: 
It's important to know that Xeon CPUs bring no benefit for DAW use.
They run at significantly slower clock-speed.
Note: Dual CPU motherboards often have issues with audio hardware (audio interfaces, UAD cards, etc).
For corporate server... sure.
For DAW, you can do significantly better at substantially lower cost.
 
 

Jim of course custom PC's have their uses (been here before), and that's generally when you want to run 128 zillion tracks on Reaktor or something..
 
Anyway fair enough Jim, every single off the shelf PC is crap, apart from yours, and none of us know how to spec up requirements. Everybody must have a custom assembled formula 1 car to run a DAW and minesweeper. And adding a separate convolution reverb on every single track the sensible and professional thing to do esp for home users. Intel business management software does not slow down PC's and all latency issues are hardware issues...
I'll leave it at that 

Jim didn't say that, doc.

Also, I use one of Jim's computers. I like it.