A separate graphics card can improve the performance, when you want to play everything higher than 16 bit console games (PSX, N64, Dreamcast, PSP, 3DS, Gamecube/Wii, PS2) at a higher native/internal resolution (upscaled).
Here you can see a list of different hardware, and which multiple native resolution did run at full speed with them:
Emulation depends mostly on Single Thread CPU Performance. The higher the Single Thread Performance of the CPU, the better.
But for some graphically more demanding games (and especially if you want to upscale the internal resolution), the GPU-performance is also important.