The wild pokemon are pretty much from the original 151 and the 100 additions that G/S/C introduced. You can catch pokemon from the 3rd and 4th gens in the Safari Zone after you beat the Elite 4 (and they've added an "item system" that makes it so you add certain items to an area, and a certain pokemon now apears in it). Maybe in some other certain areas too that are kind've out of the way, like Stark Mountain.
Not to mention new items, moves, and abilities they've added that aren't in previous versions. I'm liking SoulSilver a lot.