The 2 GB of vram is worth it imo. I see vram useage on my 7870 above 1 gb fairly often in a broad spectrum of games. Not a lot above 1 gb, but still.
For $30 that is a good deal, and will keep you happier with the card a lot longer.
a 7790 will not fully utilize that 2gb as games that are demanding like BF4 will require more graphics horsepower as opposed to vram density. there're instances though that some games do benefit from extra vram.
the beauty of pc gaming is that you can optimize your detail settings to hit the sweet spot to make that cheaper 1GB work for you.