Hey Gamer! Let me first tell you that while looking for a gaming graphic card, never look for any 4 GB variant, I will tell you the reason for it afterwards. First of all, you should know certain terms pertaining to the subject like FLOPS, Bandwidth, Texture rate and Pixel rate.
Texture Rate: You may have seen this term more frequently used while explaining the specs sheet of a graphic card. Well, as you know every game is developed out of complex coding which species several things to the compiler. So, texture rate of a GPU is the speed at which textures would be applied to the objects. So, if this rate is high then greater would be the GPU performance.
Pixel Rate As the name suggests, Pixel rate simply tells us about how many pixels would be processed per second. That is, if this rate is high then GPU can support greater screen resolutions.
FLOPS: Now, the coded data with which the developers make their program is simply a raw data. So, FLOPS tells us the speed at which GPU can convert this raw data into some processable data, after which other operations like texture operations are applied to this data.
Memory Bandwidth: By this step, you may have known how much complicated processes occur in a GPU. While performing all these operations, GPU temporary stores all these data into its memory. So, Memory Bandwidth of a GPU tells us the speed at which it will access its memory for retrieving all the stored information.
As all data is stored and retrieved temporarily (which happens in less than a micro second), so a GPU in general never uses its all available memory and in most of the recent games, it has been found that having a 1 GB variant is far more better than wasting money for a 4 GB variant (unless you opt for 4k ultra HD gaming).
So, for selecting a good gaming graphic card, always keep in mind the terms which I stated above. The first thing which you should see is the Texture rate. Then look for the Memory Bandwidth and at last FLOPS. And by the way, also look for the architecture on which the GPU is based upon. For example: the latest entry from Nvidia's GTX 960 which has just 112 GB/sec Bandwidth is faster than the ATI's R9 270X which boasts of its 179 GB/sec bandwidth. This is because former is based on the much efficient GM206 Maxwell architecture while latter is not so efficient with its GCN architecture.