you wont find a better upgrade for $50. i'd go for it. The i5 2400 can still run new games. after this upgrade, save up for a new gpu, and then upgrade the rest of the system when you are able to.
What about the extra 4GB of RAM? $50 isnt bad for the i5, but considering you'll probably need to upgrade again when you get a newer GPU, I don't know if it's worth it or not.
New pc use ddr4 ram, so you have to plan how long will you keep using your system before buying new pc.
Keep in mind this is a 6 years old system where parts may die soon