I'm wondering that what would be the escape velocity of our Galaxy?
500+ kps. There's a pretty good answer over at https://space.stackexchange.com/questions/3948/escape-velocity-for-our-galaxy, but the question appears to fit here as well.
For a mass all located inside a circular orbit, escape velocity is a simple function of orbital velocity, but that ignores most of the mass outside that circular orbit (all of it if its distribution is spherically symmetric). The Milky Way has a lot of mass outside the Sun's orbit, though.