In terms of overall gaming performance, the Intel Core i7-4820K 4-Core 3.70GHz is massively better than the Intel Xeon E3-1230 v2 when it comes to running the latest games. This also means it will be less likely to bottleneck more powerful GPUs, allowing them to achieve more of their gaming performance potential.
The Core i7-4820K 4-Core was released over a year more recently than the Xeon E3-1230 v2, and so the Core i7-4820K 4-Core is likely to have better levels of support, and will be more optimized for running the latest games.
Both CPUs exhibit very powerful performance, so it probably isn't worth upgrading from one to the other, as both are capable of running even the most demanding games at the highest settings (assuming they are accompanied by equivalently powerful GPUs).
The Core i7-4820K 4-Core and the Xeon E3-1230 v2 both have 4 cores, which is not likely to be a limiting factor for gaming.
More important for gaming than the number of cores and threads is the clock rate. Problematically, unless the two CPUs are from the same family, this can only serve as a general guide and nothing like an exact comparison, because the clock cycles per instruction (CPI) will vary so much.
The Core i7-4820K 4-Core and Xeon E3-1230 v2 are not from the same family of CPUs, so their clock speeds are by no means directly comparable. Bear in mind, then, that while the Core i7-4820K 4-Core has a 0.4 GHz faster frequency, this is not always an indicator that it will be superior in performance, despite frequency being crucial when trying to avoid GPU bottlenecking. In this case, however, the difference is enough that it possibly indicates the superiority of the .
Aside from the clock rate, the next-most important CPU features for PC game performance are L2 and L3 cache size. Faster than RAM, the more cache available, the more data that can be stored for lightning-fast retrieval. L1 Cache is not usually an issue anymore for gaming, with most high-end CPUs eking out about the same L1 performance, and L2 is more important than L3 - but L3 is still important if you want to reach the highest levels of performance. Bear in mind that although it is better to have a larger cache, the larger it is, the higher the latency, so a balance has to be struck.
The Core i7-4820K 4-Core and the Xeon E3-1230 v2 have the same L2 cache size, but the Core i7-4820K 4-Core has a 2 MB bigger L3 cache, so in this area, it wins out over the Xeon E3-1230 v2.
The maximum Thermal Design Power is the power in Watts that the CPU will consume in the worst case scenario. The lithography is the semiconductor manufacturing technology being used to create the CPU - the smaller this is, the more transistors that can be fit into the CPU, and the closer the connections. For both the lithography and the TDP, it is the lower the better, because a lower number means a lower amount of power is necessary to run the CPU, and consequently a lower amount of heat is produced.
The Xeon E3-1230 v2 has a 61 Watt lower Maximum TDP than the Core i7-4820K 4-Core (though they were created with the same size 22 nm manufacturing technology). What this means is the Xeon E3-1230 v2 will consume significantly less power and consequently produce less heat, enabling more prolonged computational tasks with fewer adverse effects. This will lower your yearly electricity bill significantly, as well as prevent you from having to invest in extra cooling mechanisms (unless you overclock).