Basically wanting to upgrade to a new computer/Audio interface . (Sonar Platinum)

Of course one of the builders who hang out here would be the best.
 
For off the shelf, I've used Sony and HP with success.  Just buy a 7200 rpm harddrive, insert it and you are good to go.  And more RAM.
 
The audio interface - how much money do you see yourself spending and how many tracks do you need to record at once?  That info will usually give you the answer.
 
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Most everything I know about shopping for an interface. 
 
http://www.cactusmusic.ca/Articles/Johns%20Audio%20Interface%20blurb.rtf
 
I recently upgraded my DAW. I first looked at the pre built DAWs which are $1,200- $3,000 price range. I then looked at all the major brands of pre built systems which are actually not a heck of a lot less than the custom built DAW machines if you go with the same specs. Most consumer desktops are minimal specs and AMD processors. So if you think you'll save money Consumer VS Custom built think again. 
 
The only way to save money is to do it yourself like many of us here have done successfully year after year. 
You can build a very nice DAW computer for just under $1,000. You can build an very nice office computer for under $500. It takes time and research but is not that big a deal for the average nerd. 
Another option as example is (if)  there is a computer shop in town and they can build custom and test it out and only charges an extra $150. 
 
 
 
 

 
 
The only hard part is quessing how much torque to apply to the CPU heatsink screws.
 
 
 

 
Are you going to be adding any PCIe cards in the future ?  You've got 6 slots.  Multiple drives ?
If so, I think your power supply is kind of weak.
Actually, I find it weak anyway, but it'll probably work as your board looks pretty efficient.
 

You might want to post in the Computer forum as that is the best place for asking about building, The question has been asked a zillion time however so you might just be better off reading all the posts. You'll get a real good idea of what people recommend including few great tips from the DAW builders themselves. When they speak,,, listen. 

USB 3 is, for the most part, redundant for audio and are thin on the ground (or store).  The "best buy" for quality and cost is the TASCAM UH-7000, which at $400 is as good as you are going to get as far as sound quality.  Pro preamps and conversion.  The downside is high latency on the usb 2 drivers and it don't look they will get an update.  If you want similar quality (w/o preamps) the new lynx pci card - $700 to $1000 for a 2x2 or 4x4 unit each with analog and digital ins/outs.  RME is another contender (good hardware and the best drivers).
 
But I imagine you are looking for something cheaper.  Focusrite is a favorite here on the lower end, tho I expect the new TASCAM series will be good (Gibson owns Tascam and Cake), and Roland (previous owners of Cake) is another good choice.  At the same price range all the hardware is about the same, except for drivers (and all the above companies have good drivers except the UH-7000, which are moderate.
 
I don't build computers - but pci slots are still valuable.  They are like a van - they don't do you any good except when you do need them, and then nothing else will do.  Try loading your mini with the show's gear.  And power - for a few dollars more you are doing the best thing possible for the long-term service of your computer.  Put something in those slots and you'll need more power, and for music itself the power supply is one of the differences between consumer and pro units. 
 
have fun