If you can be a little patient in saving a little more money for a build, that's the best way to go. Most off-the-shelf PC's use very cheap/generic parts that are likely to cause a lot of headaches with all the demands of a DAW.
The good thing about building a machine is you have complete control of everything that goes into it and if you choose wisely, you'll have a solid machine that'll last for quite some time. I'm not sure how good/bad all those parts are that you've listed above, but it's always good to research/read reviews on each to know the good/bad before buying.
If possible, add another 8Gb stick of RAM and also look into having some seperate drives (both are 'inexpensive'):
Drive 1) for Win 8.1 + all your software (an 250GB SSD is highly recommended)
Drive 2) for all your Sonar projects
Drive 3) (optional) if you have a lot of audio samples
The i7's are known to be better suited for DAW's, but I know budget comes into play.......
Hopefully, someone who's familiar with those listed parts (above) will come and chime in.
Good luck with the build!!