its only moderately better than the 7770 at an extensive cost. What it lacks most is that the card cannot be used in SLI, so in the end, if you ever decided to upgrade, you can only use it as a physx card or sell it. as for size, Kepler this generation was built to me small.
on a side note, I can almost assure that a 1gb 7850 is doable with the Antec VP
its rails are 18 18, with a max load of 360w
back in december 2011, toms 500$ budget build used an i5-2400 and 6870(which consumes more power than the 7850) using an antec earthwatts 430w
which has a 12v rail of 17 and 16, with a max load of 384w(24w difference)
using this chart
the 1gb 7850 has considerably less power consumption than the 6870(and thats reference, aftermarket may consume more power) Which makes me conclude that its highly possible to run a 7850 on a vp