Yep, the 4770 is just a hair slower than the 4850 (it is on the 40nm process and has less shaders, but they are clocked higher so the 4770 is only a hair behind in the end) so it will be a large improvement over a 2900XT.
And also, you will love the power (and heat) difference. Going from my 2900XT (which is what, 200+ watts?) to a 4850 (110W) made my comp so much cooler and happier. A 4770 only consumes in the 80W (or less, I'm not sure) range.