I agree you need to look to improve at RB. I also agree that this is a fair deal to the extent that Garcon has overachieved so far and Greene would at least be someone getting regular carries in return for a WR who could easily quickly regress to WR3 type status based on a shaky QB situation.
That said, I still wouldn't do it. I have more faith in Garcon's ability to continue to outperform his draft expectations than I do Greene's ability to not be a bum, ever. Greene is the epitome of a guy who should be seizing a prime job on a good team with a coach that likes to run, and instead keeps not being able to fend off a veteran (albeit a future hall-of-fame one).
My guess is you're actually likely to get more people telling you to take this than not, and maybe I'm just biased having been burned by Greene before, but I'd look for other options.