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.
I hear rig lists are all the rage, and I <3 the rage! Workstation = Black tower thing; HTPC = Shhhh!; Laptop - AMAZING FOLDING PC!