what if I mine Eth with 2gb GPU ?? Will it not work ?
It may now, but one of Ethereum's defenses against asics was the increasing size of the DAG file. I believe this all has to fit within the GPU's memory, and keep in mind this is not one to one as there is other information that also needs to be in the GPU memory at the same time. So while the current DAG, which is about 1.3 GB and even with the overhead should comfortably fit with 2GB of GPU memory, at some point the file will become large enough that you would need a 3GB card to run it.
If you are looking to buy a GPU for gaming and to mine things like ETH on the side, yes it can be a bit of extra income. If you are looking solely to profit off ETH by buying new hardware and expecting to pay it off and keep on earning profit afterwards, you may be disappointed.