How are these computers? My friend recommended them to me.
http://www.newegg.ca/Product/Product.aspx?Item=N82E16883229235
^Compared to the one I linked, this one comes with an inferior CPU and GPU, as well as an uglier case. :( It has 6 GB of RAM vs the 4 in the one I linked, but it is slower memory and the extra 2GB won't help you really. RAM is also cheap to upgrade. You can upgrade to 8GB on the NCIX built for +$46 if you really want. I guess more RAM is cool, but I don't think it's necessary. The Newegg build does come with a keyboard and mouse, but meh.
http://www.newegg.ca/Product/Product.aspx?Item=N82E16883229235
^Same as above.
http://www.newegg.ca/Product/Product.aspx?Item=N82E16883227324
^Good CPU, but it is the locked version (doesn't have the K in the model name) which means that you cannot overclock it if you later decide you'd like to. The latest Intel CPUs overclock -very- nicely, so I think it'd be a shame. 2500K > 2600 imo. That Newegg build comes with a faster GPU (6870 vs 6850) but it costs more. You can upgrade the NCIX build to a 6870 for +$80, and it would cost less than the Newegg build.
http://www.newegg.ca/Product/Product.aspx?Item=N82E16883229235
^Same as comments for #1 and #2
Basically, there is no reason to go for something lower performance than the NCIX build, unless you're aiming to save money. And remember that I am assuming you will do the 2500K CPU upgrade on the NCIX build in these comparisons.