I don't know the first thing about building a computer but like most other things I assume you will get more for your money if you do it yourself. Especially with your budget.
That is a solid deal def better than anything local and it always shot fine out of my gun. I stocked up on it last year when I found it for 18.99 a box.
Is this http://www.cheaperthandirt.com/product/MAG-068?utm_source=EmailDirect.com&utm_medium=Email&utm_campaign=Magazines+-+1%2f15+Campaign
They show in stock as well