Although there are others here on this forum , like Eric Bowen at ADK, who know much more than me.......I can say that you should take advantage of the new X99 based Haswell E based technology....forget Apple products. AE uses all the memory you can throw at it,so, an 8 core i7 5960X CPU, overclocked to above 4Ghz and as much new Quad-channel system RAM as you can afford would be a good start. Brand new high-speed storage options are just being released...the NVMe PCI SSDs. With read speeds of over 2GB/sec. and write speeds over 1.5GB/ sec, you would want a brand new MOBO which can support that. Use at LEAST 64GB of system RAM. The X99 MOBOS have up to TEN SATA III ports for connecting many SSDs in a RAID 0 for your media. All else would go on the NVMe SSD. Although Premiere Pro makes tremendous use of NVidia GTX GPUs....AE makes almost NO use of the GPU for acceleration.....but, a large GPU memory is good for 4K and above......get at LEAST a 970 with at least 4GB video memory.....I think the new Titan has 12GB....if you have the dough.
Premiere Pro has made huge improvements in performance, due to better use of memory and other changes.....I do not know if AE has kept up. The hardware certainly has made massive leaps forward in the past three years and it shows no sign of stopping. I would maybe wait a little to insure you may get an NVMe compatible MOBO. HP is working on releasing their version of an integrated NVMe SSD in their desktops.....others must be doing the same, as we speak.
Sorry you old rig was stolen.
Yes, it would be a dramatic speed improvement to go with a Haswell-E PC!
An overclocked i7-5960x would be awesome and much faster than you old Mac (4 GHz is easy, 4.4 doable with a bit more work). AE loves RAM so 64GB would be even faster, but DDR4 is $$$ now so putting in 4x8GB sticks now would be a great way to start and provide some headroom to grow to 64GB if required.
Premiere Pro does wonderful things with a GTX video card. AE doesn't use your video card for much though.