ricoskyl
I think everyone's advice here is right on.  I upgraded recently to an i7 HP laptop with a touch screen.  The only regret was that I didn't pop for SSD storage.  I will probably add that (outboard), but because I often work away from the studio and I'd like to keep the footprint small, but I could have also added external HD storage and kept the SSD on the main chassis. That might have been a better choice for me.
 
I know this all doesn't apply directly to your situation, but I hope it helps.  Having spent considerably more on a CTO laptop, I'd be interested in hear what you end up with and how it works out for you.  You probably already know that your $ gets a lot more than it did 5 years ago.  Generally, I support Mesh and SuperG in suggesting that you avoid skimping here; even if you build a strong system today, your needs and uses for it will almost certainly increase so it';s a little like moving backward as technology advances.

Speaking of external HD's, I'm really impressed with the USB-3 Seagate Passports. Compared to USB-2 drives those little suckers are faaasst! They're small enough to fit in your pocket!
 
 

I've just gotta get up to speed on the newer generation of hardware. Everything changes so fast. I will echo the other's sentiments about choosing parts. The pain you feel now with stretching the budget a little will be forgotten soon enough, but the awesomeness will go on and on and on.

This is almost a repeat from another thread.
The OS and programs should be on a Raid-0 SSD array
Then have two 1TB spinner drives, one for samples, one for Audio.
 
For a single threaded program (I know Sonar is multi-threaded) The speed of a computer = (Clock/access time) - whatever load the video card dumps on the CPU.
Since clock speeds are maxed in the 3-5 Ghz range (including overclocked machines) the best improvement is reducing the seek times with Raid-0 solid state drives. But they are still too expensive for the space needed for samples and the actual files. Sonar can be set up to use separate drives for samples and Audio. Just keep the program and OS on the main SSD's
 
I'm not current on mother boards, but some are better for Audio than others. I would try to find out what PC labs and other manufactures of Audio only computers are using and use that.
 
That seed, There are cost effective drives out there that may have reliability issues but are cost effective. 12 gigs of ram is enough, and an I5 will be enough to get going. But you should try to get a socket that can be upgraded to an I7 later. At that budget, you are better off getting faster drives and using built in vid cards. do not get a shared memory vid card. You will want all the ram for the software.
 
If you really don't have the budge for multiple hard drives, there are new hybrid drives with both SS and spinner in the same drive.
 

I would also second maybe springing for an i7 CPU. It's not much more (depending on other requirements) and the more future-proofing the better. I'm very impressed by the cost of your build though!

Several reviews on the cooler master fan say  the fan can be loud. Several here are using the i5 with no worries on smaller projects.
 
Good idea to add the GPU if later necessary.