Well for most games that only need single thread performance there is nothing much to gain with the I3 2100, As you say i3 has hyper threading. Many games are being developed that will use more cores and some MMOs are moving in that direction.
The Sandy bridge i3 2100 has 2 cores and not a very powerful CPU. The g3258 is at least OC-able. If you want to stay with Socket LGA 1155 get the i3 3250.
Here is a thread that debates the issue: http://www.tomshardware.com/answers/id-2674115/pentium-g3258-2120-budget-gaming.html
If I were you and wanted good gaming and MMO experience, Save your pennies and upgrade.