At 1080p you don't need more than 1GB of VRAM yet.
You also don't need more than 3-4GB of system RAM either.
Neither upgrade will make any difference to you at the moment, but if you upgrade to a 4MP display that 1GB is going to choke, whilst a machine with 8GB will only be faster than a machine with 4GB if you heavily mutlitask, which is unlikely when you're gaming.
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