It will outperform 6870 and have the same performance level as the 6950 1GB card in some cases that the Application copes well with the Crossfire Configuration. But in games that doesn't cope at all with the crossfire, you will not even see an 'upgrade' from your single 6770 configuration. So, it's better to research your games first before proceeding. But anyway, in my opinion getting 6870 and then crossfire it later is a better option than this, since 6770 is a mid-range card.
By the way, I like how you post picture of your card here, it's a really really nice idea to do that and I haven't seen them before in tomshardware
Also make sure you have an enough power supply in case you want an HD 6870 crossfired, if no you must upgrade it.