From a pure value standpoint, it's a great trade for you. I rank Davis and Gates pretty equal right now, believe it or not. Wallace is the #4 option in that passing game howveer (after Ward, Holmes and Miller), so obviously you win out big landing a no. 1 WR like Smith.
I'd make the deal, but I'd try to flip Smith to someone else right after. I don't expect much from him the rest of the year with DelHomme throwing to him, and that offense is all about the run, so you won't be getting the Steve Smith we are used to seeing.