CPU or GPU upgrade?

mccloud182

Distinguished
Jan 21, 2012
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I'm looking to upgrade either my CPU or GPU but I keep waffling between the two, any thoughts would be welcome.

CPU- I5 2400
GPU- GTX 660
Mobo- ASUS P8Z77-V LK
Ram- 16GB DDR3
PSU- Thermaltake 500w

Typical games played:
BF4, Diablo 3, various mobas

I have a budget of around $400 US. Your help would be appreciated.