Xeon and Opteron processors are the ONLY modern x86 processors on the market capable of running in tandem. All other CPU brands (Celeron, Pentium, Core iX, Sempron, Athlon, FX, AX) are incapable of running on tandem motherboards.
That being said, it would not be unreasonable to make an entry-level gaming PC based on the G1840 CPU. Because you are looking at the absolute cheapest CPU available on a modern socket, I am going to assume extreme budget constraint - you could do something like this, using SteamOS or Linux as your OS to get started with gaming:
- CPU: Intel Celeron G1840 2.8GHz Dual-Core Processor ($39.99 @ SuperBiiz)
- Motherboard: Biostar B85MG Micro ATX LGA1150 Motherboard ($47.98 @ Newegg)
- Memory: PNY Anarchy 8GB (2 x 4GB) DDR3-1600 Memory ($23.98 @ Newegg)
- Storage: Western Digital Caviar Blue 250GB 3.5" 7200RPM Internal Hard Drive ($22.25 @ Amazon)
- Video Card: Zotac GeForce GTX 750 Ti 2GB Video Card ($99.99 @ Newegg)
- Case: DIYPC M89-R MicroATX Mini Tower Case ($23.99 @ Newegg)
- Power Supply: Raidmax 380W ATX Power Supply ($19.99 @ SuperBiiz) Total: $278.17
Alternatively, you might consider looking into an AMD APU option, as most of their A6, A8, and A10 desktop processors are vastly more powerful than the Celeron G1840, and come with integrated graphics that are actually useful for light gaming. For example, the A8-7600 is much faster than this Celeron on average, yet it also features an integrated GPU capable of comfortably handling games like CS:GO, League of Legends, or most Rocket League at high settings and framerates at 1080p.