Here's an extensive thread where many games were tested with the G3258.
http://www.overclock.net/t/1500524/intel-pentium-g3258-performance-and-owners-thread
In summary, it does pretty well in most games; however, some of the more CPU dependent and multithreaded games do not perform so well.
Even in modern games, it beats an Athlon x4 750k, which is faster than your Athlon, so it would give you a performance boost for potentially a cheap price.
http://www.tomshardware.com/reviews/pentium-g3258-overclocking-performance,3849.html
To be more future-proof in general gaming, I would recommend a 4-core i5 processor. Ironically, this may perform a bit worse in Dolphin if it's clockspeed does not get as high as the G3258 (but it should be enough).
http://www.overclock.net/t/1500524/intel-pentium-g3258-performance-and-owners-thread
In summary, it does pretty well in most games; however, some of the more CPU dependent and multithreaded games do not perform so well.
Even in modern games, it beats an Athlon x4 750k, which is faster than your Athlon, so it would give you a performance boost for potentially a cheap price.
http://www.tomshardware.com/reviews/pentium-g3258-overclocking-performance,3849.html
To be more future-proof in general gaming, I would recommend a 4-core i5 processor. Ironically, this may perform a bit worse in Dolphin if it's clockspeed does not get as high as the G3258 (but it should be enough).