Card from AMD are better for dolphin, specially 6xxx Familly, but for OGL, Nvidia got better driver for Linux, if you are planing to use under windows on OGL, AMD card get almost same perf than NVIDIA. Only AMD linux driver are bad. (By bad i mean , horrible)
I don't think the HD 7750 has OpenGL 4.4, which Dolphin just implemented some new speed-increasing extensions in.
The GT 640 should also have a higher fill rate and bandwidth, which Dolphin likes
The 7750 has pinned memory, which is just as good as buffer_storage (the 4.4 stuff you're thinking of). Also, AMD drivers for OpenGL 4.4 are really close now, and the 7750 is 4.4 compatible.