Moving this discussion to the Hardware forum.
Mike you can find the system requirements for Photoshop CC 2014 and Premiere Pro CC 2014 referenced at System requirements | Creative Cloud. The community in the Hardware forum can provide more specific suggestions.
I would suggest atleast 16GB of ram with 32GB really what you want because of the dual GPU''s. If you don't have enough ram to feed both video cards then both those cards will have limited performance. The D300 cards are really a waste. I wouldn't bother with anything less than the D500 cards especially if you plan to use Premiere. AMD cards already have half the performance that Nvidia cards do with Adobe GPU acceleration in Premiere. I wouldn't make it worse with the D300 cards. I would also suggest the 6 core CPU over the 4 Core. Otherwise just get the new Imac instead of you want a Mac.
...it may not be a bad idea to look seriously at the lower cost, the higher performing, and the more compatable Windows machines for use with PPro and Ps.....
Many users have migrated to Windows machines in light of the superior performance and lower cost...AND more options when configuring the machine to achieve best performance at a better price.