Should I upgrade my i3-2100?

Not really necessary. The i3 can easily handle the GTX 650 Ti.

If you are playing games that are CPU bound, then an upgrade can give you better performance, but it generally will not be a huge increase. Performance increase depends on the game. For example, Skyrim stresses both CPU and GPU so if you were to upgrade to a faster CPU then you can get a little better performance.

Let's say your motherboard is compatible with the 3rd generation Ivy Bridge Core i5-3450 which runs at 3.5GHz. That is 400MHz faster than your 2nd generation Sandy Bridge Core i3-2100 CPU running at 3.1GHz. Additionally, Ivy Bridge is about 6% on average more powerful than Sandy Bridge assuming the same clockspeed. Also, I do not believe Skyrim uses more than 2 cores. Given the above info, the 3rd gen Ivy Bridge i5-3450 would be roughly equal to a 2nd gen Sandy Bridge Core i3 CPU at 3.7GHz. That is almost a 20% increase in raw processing power, but raw processing power does not directly translate to the same amount of performance increase. So you may only get a 10% - 15% boost in actual performance depending on the game/program.

If Skyrim is capable of using 4 cores effectively, then upgrading to a Core i5-3450 would provide a significant improvement in performance. How much? Hard to say since it depends on how well games/programs can use more than 2 cores. You will definitely not see double the performance. More like a potential 20% - 40% (on the high end of expectations) increase in performance depending on how efficiently the game / program has been designed to use more than 2 cores.