The i3-3420 is a decent gamer.
If you want to game, a graphics upgrade is in order.
The optiflex comes in three form factors.
If you have one of the low profile versions, you are going to need a low profile graphics card.
The second limitation will be your psu which will be in the 240w range.
A GTX1050ti will be the strongest you can run
so long as the card does not need aux 6 pin power.
Here is an example which should come with a low profile adapter: https://www.newegg.com/Product/Product.aspx?Item=N82E16814137081
You Need a new CPU,GPU and a PSU to call it UPGRADE because getting a NEW GPU will bottleneck your current CPU plus your stock PSU probably isn't powerful enough to provide enough power to NEW GPU.
The i3-3420 is a decent gamer.
If you want to game, a graphics upgrade is in order.
The optiflex comes in three form factors.
If you have one of the low profile versions, you are going to need a low profile graphics card.
The second limitation will be your psu which will be in the 240w range.
A GTX1050ti will be the strongest you can run
so long as the card does not need aux 6 pin power.
Here is an example which should come with a low profile adapter: https://www.newegg.com/Product/Product.aspx?Item=N82E16814137081
The maximum wattage for the pci-ex slot in a 3010 SFF is 50W, so I'd stick with a GT 1030 DDR5 card since it doesn't use that much power. You'll still be able to game on it, just certain games might be out of the question.
A lot of people report ignoring the Dell PCIe power limit without issue. But the risk is on you of course.
Anything higher than a GTX1050Ti will require a 6 pin PCIe cable which doesn't exist.
With 8GB RAM I would get the 2GB GTX1050.
Here are the builds for that. You can look at the CPU scores, and RAM capacity. But the big GPUs will be in the tower cases with aftermarket PSUs. But the GTX1050 score will be relevant. http://www.userbenchmark.com/System/Dell-OptiPlex-3010/2323