Is i worth the extra 50? My budget is pretty spent at 800 already. My main purpose is gaming, but nothing too serious like Crysis. I'll be playing things like Heroes of Newerth, Grid, Dirt 2, Just Cause 2 and what not.
And who said the P7P55D Asus is obsolete? Maybe not available but definitely not obsolete. Some say that socket 1156 is a dead end, though specially if you pair it with an i5 750. It depends on who's talking.
IMO, the 550 watter is a bit light for twin GFX cards and given your selection of the 5770, if you move to playing DX11 at 1920 x 1200, you will be a bit underpowered so you'll want the option to add a 2nd one in XFire.