Apple iMac with Retina 5K display at Amazon for pre-order at standard $2499, tax in most states but free shipping with prime plus options for a 4 year warranty for just $120.59 more. Upgrade the RAM yourself with the Crucial 16GB Kit (8GBx2), buy two for 32 GB. (vs paying $600! to upgrade from Apple).
The i7 Quadcore looks good at $250 more, but is currently only available through Apple. Give it a few months and that configuration should be available elsewhere.