Small and Practical Linux Box

You could also just get an FX 8 core and do anything you want ever with it at around $400, it's even better if you don't need like a case/PSU/storage

"El Burro" The Workhorse AKA The Render Bitch

Also good for running multiple VMs

Course it won't be ITX, though you could get a compact ATX case

PCPartPicker part list: http://pcpartpicker.com/list/FhrkTH
Price breakdown by merchant: http://pcpartpicker.com/list/FhrkTH/by_merchant/

CPU: AMD FX-8300 3.3GHz 8-Core Processor ($119.88 @ OutletPC)
Motherboard: ASRock 970A-G/3.1 ATX AM3+/AM3 Motherboard ($71.99 @ Amazon)
Memory: G.Skill Value Series 16GB (2 x 8GB) DDR3-1333 Memory ($51.99 @ Newegg)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($47.49 @ OutletPC)
Video Card: XFX Radeon HD 5450 1GB Video Card ($27.99 @ SuperBiiz)
Case: Deepcool TESSERACT WH ATX Mid Tower Case ($33.99 @ SuperBiiz)
Power Supply: EVGA 500W 80+ Bronze Certified ATX Power Supply ($42.99 @ SuperBiiz)
Total: $396.32
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-07-03 23:50 EDT-0400

That's basically what my other rig already is, except I've got the Asus Strix 380x 4GB

Wait you already have 2 PCs? You could always get a cheap laptop as your third rig to simplify things

I could, but I have a 21:9 monitor I want to hook it up to. I'm not really a fan of laptops either.

My AMD 8 core PC is actually my girlfriends. She uses it for her graphic design work. I just want another one for myself to use.

I like Chopin + A8-7600 for SFF. Good compromise between low power, good cpu, good gpu. All around solid build.