I'm not that much into gaming, so it's not the power I'd need. 8800 GTS meets my gaming needs pretty much perfectly.
And I see HD5770 has VGA/DVI/DP/HDMI outputs, so I'd need to buy new display that had either DP or HDMI to run off with that. And given that the display I'm looking into has neither, it sounds kind of a bad choice.
Also I thought that my mobo doesn't have IGP O_o
But how about using the old PCI line? Those cards should be quite cheap, at least the ones that can just put image out.