Re:core i5 vs core 2 quad(Q8400)
i5 >>>> your Q8400. Be sure that you're also snagging a P55/H55 chipset board, because the i5 is incompatible with the LGA 775 socket that your current motherboard sports.
i7-4770K @ 4.0 / Gigabyte Z87X-UD4H / DDR3-1600 G.SKILL Ripjaws X
EVGA GTX 780 Ti SC / Silverstone Strider Plus 1000W
Corsair H110 / CM Storm Stryker
CM Storm Trigger / CM Sentinel Advanced
Epiphone Sheraton R62 / Vox AC15C1