I would get another stronger card rather then get another in Crossfire. I would say at least a 7850 for the upgrade to be worth it.
You would need a bigger PSU for Crossfire/SLI, though I suspect two 7750 in Crossfire would be an exception though. They sip power and dont require any power cables.
Sapphire, Gigabyte, ASUS and MSI are the brands to get. As I said above, I suggest at least a 7850 for the upgrade to be worth it, so essentially any card above ~$200.
The amount of VRAM doesn't affect performance beyond a certain point. If you don't have enough of it, performance will drop. Though if you have more than enough, performance wont improve because of it. 2GB is what most cards have and is sufficient for 1080p gaming, very few (modded) games can exceed 2GB at that resolution.
Also, you may have to upgrade your PSU anyway. Will depend on what card you pick though.