How much are you paying for that sandy i3? If you want to go with an i3, you can get a brand new i3 6100 for slightly over 100.00. (125.00 on new egg, but you might find a better deal).
The skylake i3 has almost 20% higher clockspeed (3.7 vs 3.1) and considerably better ipc (another 20% or so, maybe more). Overall, that should make a skylake i3 40 to 50 percent faster. Also, DDR4, which skylake requires, is also available at good prices now. IMO, you should seriously consider Skylake instead of used, unless you already have ram and a motherboard to pair with the 2100
Edit: confused with another thread. I see that you already have tbe 2100. Definitely go with that over the pentium. It will be decent for most games, but like i said, it is quite a bit slower than a newer i3, much less an i5. You may have to upgrade at some point, depending on the games you play.