Ok - the 7770 can support 3 out of the box, which is why I asked.
And then I guess it's up to as far as the HDMI vs. mini. All the GTX560s I've seen have a mini-HDMI, so you can either buy that and an adapter (or use DVI) or buy the 7770 which has a full-size HDMI...and doesn't perform as well.
That said, it's maybe 1000-1200 3DMarks difference.