Games on a PC are scalable with quality, so you don't need a NASA supercomputer to play if you don't want to invest that amount of money. However, you do need something reasonably powerful. Those are fairly high-end games so you will need something somewhat good.
Case: You may be able to reuse your old Dell case, however, don't expect being able to run multiple graphics cards and high-end GPU's will be too large.
Motherboard: ASUS M5A97 AM3+ AMD 970
- This is a good motherboard that is compatible with AMD's AM3 and AM3+ CPU. It also has USB 3.0, SATA 6, and UEFI. If you are going to be using more than one graphics card, you may want one of AMD's 990FX based motherboards. $94
CPU: AMD FX-6100 Zambezi 3.3GHz
- This is a good CPU for what you are doing. It isn't high-end, nor is it low-end, but it will last for a little while. If you want a higher-end CPU, I would suggest the AMD FX-8120 Zambezi 3.1GHz
. Keep in mind that Windows 7 can use this CPU quite well, however, the scheduler doesn't utilize the FX CPU's to their full potential. Windows 8 is aware of the architecture and has anywhere from 2% to 10% of a performance increase over Windows 7 using FX CPU's. $150
Power Supply: CORSAIR Builder Series CX500 V2 500W
- I highly doubt your Dell has a decent power supply. Assuming your Dell case is standard and allows for an off-the-shelf ATX power supply, this will work. $59
RAM: AMD Entertainment Edition 4GB 240-Pin DDR3 SDRAM DDR3 1600
- This should be enough for you, if you're unsure, add another stick. $20
Graphics Card: XFX HD-687A-ZHFC Radeon HD 6870 1GB
- This will work with those games quite well. You won't be able to do ultra quality, but you will be able to use high at least. $164
Operating System: Windows 7 Home Premium 64-bit OEM
- Due to licensing restrictions, you cannot reuse your copy of Windows. The license defines your PC by its motherboard, so, new motherboard, new PC. The system builder/OEM versions of Windows cannot be moved to a new PC, however, full, boxed, retail versions can. You can either buy Windows 7 now, or use the Windows 8 Consumer Preview until Windows 8 comes out and buy it then. Remember, Windows 8 CP is still a beta so your experience may vary in quality. $100
Grand Total: $590 (Before taxes, rebates, and shipping)
You should be able to reuse your optical drive and possibly hard drive, though, I would suggest replacing your hard drive if it is five years old or more.