Which gaming PC build should I do?

I do like the idea of the used part build (last one). With a $400 budget and used parts you could do one with a Core i5 or i7, 12GB to 16GB DDR3 RAM, 1TB HD (any brand other than Seagate which has an appalling failure rate), 80+ 500W PSU and a GeForce GTX 670 4GB (which will blow a GTX 950 out of the water let alone a 750 Ti).

- Base Computer: This is the foundation, the most important part. Get a used second generation or later Core i5 or i7 Desktop with Windows 7 or later for about $175. Look it up through the manufacturer first to make sure it supports SATA III, 4 RAM slots (up to 16GB or 32GB), Full height and Full Length PCI-e card. Preferably the computer will boot with a minimal hard drive and RAM installed. If the case is BTX (motherboard on left) be sure there is an open slot above the PCI-e x16 some BTX only allow single wide cards due to bad design.

- Buy DDR3 RAM used. Last I checked 8GB (2x4GB) can be had for $15 on eBay based on sold listings.

- PSU: New get a 500w Corsair or Evga 80+ Bronze PSU for about $45

- GPU: Buy used. The GTX 660Ti 2GB, GTX 670 2GB and GTX 670 4GB are the sweet spot. They outperform a GTX 950 and cost $70 to $115. The 670 4GB is obviously the best. I've seen them sell for less than $80 but $100 to $115 is more realistic if you aren't dedicated to finding a deal.

- Hard Drive: Only new $50. 1TB or higher WD, Hitachi (HGST) or Toshiba. Seagate has a horrible record lately

For about $400 you could have

- Core i5 2400 or better possibly a 3000 series

- GeForce GTX 670 4GB

- 16GB RAM

- 1TB HD

- 500W 80+ Bronze PSU

- Valid Windows 7 through 10 license