I've been thinking that it's actually worth, but I'm pretty sure I'm missing something there, as I looked into the last AMD 8x R9 390, that has a 250MH/s rate, which is pretty impressive, while it only costs 300€. So, shall I go for this GPU ?
for the cpu mining i suggest you to read this post : Is CPU mining even worth the Ether?
however before you buy a gpu to mine ethers consider to get an estimation about the Profitability , there is a lot of tool to do so i suggest you to try : https://badmofo.github.io/ethereum-mining-calculator/ or https://etherscan.io/ether-mining-calculator
also you could get an idea about the hasrate comparaison between the GPUs used to mine ethereum : http://www.mininghwcomparison.com/list/index.php?brand=amd
i made the calculation i've found that you will get about 29(>300$) monthly.