Mohammed Allam :
When u think i should upgrade
When you run into a game that does not run well enough for you to enjoy it and trace the bottleneck to the CPU. In general, if you need to ask if you should upgrade, the answer is no.
When I had my Core2Duo with 8GB RAM, I thought I would tough it out until Haswell came out but for the sort of workload I was dealing with at the time, I really needed at least 16GB RAM. I ended up building around an i5-3470 and put 16GB in it, that did wonders for my sanity but was still not quite enough to eliminate swapping so I bumped it to 32GB RAM two months later.
When you actually NEED to upgrade, you NEED to upgrade. (And you won't need to ask if you need to or should.)