1600pro has 12 pixel shaders and five vertex shaders that feed four pipelines.
3850 has320 universal shader units, 16 ROPs, and a 256-bit memory bus.
You will notice a big improvement over every single game you play, just dont expect miracles as your P4 will bottleneck any cpu intensive or multi-core threaded game on the market, games such as GTA4 need plenty cpu power so there the gains will not be so big, but in Assasins creed you will be jumping a huge step in performance....
Read this http://www.tomshardware.com/reviews/ati-agp-3850-agp,1939.html
And then make up your ming if spending some dosh now will give you what you want, or do you save for new build!