Well the 450 is certainly a huge improvement over the 4770. So yes, it would theoretically improve graphics performance. At your resolution however? Not massively, i think 1600x900 won't tax too many cards unless you really start ramping up the settings.
If you want to get the 450, go right ahead, you may be able to set some game settings to higher etc without compromising framerate and will be able to make use of DX11 if you're on windows 7 (bad company 2 etc).
On the other hand, if you save your money further for a more meaningful upgrade to the rig as a whole and a bigger monitor, you will have a much improved experience over a simple GPU upgrade to a monitor that doesn't necessarily need one.