If you're budget conscious, you can't go wrong with as Intel Core i3-7350K Kaby Lake Dual-Core 4.2 GHz for around $150.
The stock clock is 4.2GHz, which is good for MAME.
It only has dual cores, but it has hyperthreading.
If you want more cores, you can always go with one of the i5/i7 CPUs.
For motherboards, get something from ASUS.
Here's a decent budget barebones (motherboard, cpu, cooler, memory, case, psu) system...
PCPartPicker part list: https://pcpartpicker.com/list/vkgx8K
Price breakdown by merchant: https://pcpartpicker.com/list/vkgx8K/by_merchant/
CPU: Intel - Core i3-7350K 4.2GHz Dual-Core Processor ($140.99 @ Newegg Marketplace)
CPU Cooler: Cooler Master - Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler ($19.99 @ Newegg)
Motherboard: Asus - H110M-E/M.2 Micro ATX LGA1151 Motherboard ($59.89 @ OutletPC)
Memory: G.Skill - Ripjaws V Series 16GB (2 x 8GB) DDR4-2133 Memory ($145.98 @ OutletPC)
Case: Silverstone - TJ08B-E MicroATX Mini Tower Case ($98.99 @ SuperBiiz)
Power Supply: SeaSonic - 520W 80+ Bronze Certified Fully-Modular ATX Power Supply ($51.99 @ SuperBiiz)
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-10-11 09:58 EDT-0400
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