If you CFX with a single slot card and dual slot card of the same type, then there wouldn't be any performance difference, the dual slot sometimes has higher clocks, but the majority of the time, they are the same. The cards you suggested are actually dual slot because of the cooler which partially blocks the slot directly underneath the fan, however this is not a problem if your expansion card is short. Also, both HD 5770s should support OpenGL, except that XFX may not have shown such information on their website.