I'm exactly that user you're dismissing when you say that someone with a big monitor isn't shopping for a $600 gaming PC. I have a $350 refurbished Auria EQ276W I picked up from Microcenter. I can't find a color calibration that I like, but it's 2560x1440 with DisplayPort, HDMI, and DL-DVI for cheap! There are other options out there too, especially for hooking up to a gaming PC, that can get you a 2560x1440 IPS panel for a LOT less than the Dell or HP screens.
When I sit down to price out gaming PCs for Mechwarrior Online, SC2 Heart of the Swarm, Skyrim, etc., your value PCs are exactly what I look at.
However, I would also always, always put at least a 32 GB SSD in any system I would build.
So I think building a $750 gaming PC with an SSD and focused on 2560x1440 gaming performance would be extremely interesting. Double bonus if you can keep it quiet and stuff it in a mini ITX case.