1. i3 2120 cannot be overclocked. Overclocking is not supported in that.
2. The LGA 1155 socket for the i3 2120 CPU is End-Of-Line. That is no more new CPU's would be launched for it. Hence no upgrades possible. The only possible upgrade is to either to i5 3470 or i7 4770. Nothing more than that.
3. Also do you already have the CPU or you are buying it?
Anyways, the best budget board for this build would be :
PCPartPicker part list /
Price breakdown by merchant /
Benchmarks
Motherboard: MSI B75MA-E33 Micro ATX LGA1155 Motherboard ($56.99 @ Mac Mall)
Total: $56.99
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-12-17 13:16 EST-0500)
But anyways I don't recommend going Socket LGA 1155. It is dead. End-Of-Line. No new launches for it anymore. So you would be like stuck forever.