OK, it must be said that your revised $400 budget is still a fairly low budget for a gaming PC. But, given that you play PayDay 2, I'll use its recommended specs as a general benchmark as to what sort of specs you should have. For $350 after rebates, you can get these parts:
PCPartPicker part list / Price breakdown by merchant
| Type | Item | Price |
|---|---|---|
| CPU | AMD 5350 2.05Ghz Quad-Core Processor | $54.99 @ NCIX US |
| Motherboard | Gigabyte GA-AM1M-S2P Micro ATX AM1 Motherboard | $26.98 @ Newegg |
| Memory | Crucial 4GB (1 x 4GB) DDR3-1600 Memory | $29.25 @ SuperBiiz |
| Storage | Western Digital RE3 250GB 3.5" 7200RPM Internal Hard Drive | $24.99 @ Amazon |
| Video Card | Asus GeForce GT 630 2GB Video Card | $29.99 @ SuperBiiz |
| Case | NZXT Source 210 (White) ATX Mid Tower Case | $33.99 @ SuperBiiz |
| Power Supply | Rosewill Capstone 450W 80+ Gold Certified ATX Power Supply | $59.99 @ Newegg |
| Operating System | Microsoft Windows 8.1 (OEM) (64-bit) | $89.98 @ OutletPC |
| Total | ||
| Prices include shipping, taxes, and discounts when available | $350.16 | |
| Generated by PCPartPicker 2015-01-15 22:31 EST-0500 |
That will give you sufficient performance for what you play and many other games. Of course, there are a few places you can shave a few bucks, namely with the case, HDD, and GPU. However, the CPU, mobo, and RAM are about the cheapest you can go while still being able to run what you want and get decent quality parts. That GPU should be sufficient for your needs, but bear in mind that it's not exactly a powerhouse. The PSU is very high quality and very efficient, which is what you want because that is really the heart of your PC. I always recommend NZXT cases and the Source 210 is a great choice for budget PCs. The black version is cheaper. And finally, OS choice is really down to you, but I like Win8.1. If you want, you can run Linux or some other cheap/free OS that is compatible with what you want and save some money there. Maybe invest it in better parts.