Question What is the best CPU I can upgrade my system too?

[DXXXX]

I have 265 wz power supply (getting a cx450 soon), OptiPlex 390 case, 0M5DCD Motherboard, 8gb ram, and a i5 2400

What CPU can I upgrade too that is the best? (and that will work on my system of course)

 

[SkyNetRising]

What CPU can I upgrade too that is the best? (and that will work on my system of course)

i7-2600k.
With proper bios update also i7-3770k.