Sorry, but no, you're not making a Mac Pro. You're using a consumer CPU, consumer RAM, a *single* consumer GPU. That's of course going to be cheaper, and then putting aside the fact this machine will be large, loud and generate a lot of heat.

Good enough for most though I expect. But did you really imply that the difference between the MacPro's CPU and a "PC" version was the chip being "branded" a Xeon? Seriously?