My Nephew is running a Q9650 overclocked to 4.2 GHz, an Asus X48 motherboard, 8 GB DDR3 1600, two SSD's in RAID0, and an overclocked GTX 1060 6GB. He says it's still a competitive gaming rig.
Here's his O/C'd PassMark CPU score compared to newer processors with similar scores:
i5 6400 - 6,550
i5 3450 - 6,448
i5 2500K - 6,438
i5 4440 - 6,404
Q9650 @ 4.2 - 6,342
FX6300 - 6,330
i5 2500 - 6,232
i7 975 - 6,214
i5 3340 - 5,998
3DMark Time Spy - 3,929
3DMark Fire Strike - 10,321
CT