Only AMD Radeons will dual graphics with the integrated graphic core. Any nVidia card will deactivate the iGPU.
From what I've read, up to a Radeon HD 7750 can dual graphics with the AMD iGPU, but the performance gains are minimal to the point where dual graphics and a standalone HD 7750 performances are almost identical. The best performance boost seems to be with a Radeon HD 6670 DDR5 graphics card.
The best budget graphics cards are the ones I mentioned above and I would choose the HD 7750.