First of all, I live in Houston and am a Texans fan, and the "word out of Houston" that someone mentions above is a little exaggerated. I agree that Chris Brown will get some carries, while healthy, and he may cost Slaton some. But I don't think he's going to be a Lendale White-style vulture, and Slaton will absolutely get red zone action.
Anyway, the original trade is crazy as others have suggested. For the original poster, here is a suggestion for looking at trades:
Think about the fantasy draft. Since this is week 1, this exercise is easy. Slaton went in the 2nd round in almost every draft, often at the beginning of the 2nd. Farve went in the 7th, 8th, 9th, 10th, 11th, etc. So that tells you what the value is.
So using that same logic, trading Slaton for a QB would mean P Manning, or maybe even Brady (although this wouldn't happen, for the reason that people who draft one of the top 3 QBs with one of their top 2 picks are enamored with them and wouldn't trade. But the point is, that's what you would be looking for).
You can use this same logic in the regular season...meaning think to yourself...if the draft were today, where would these guys get drafted? If Farve has a big first few games, then you might move him up to 4th round territory, or something like that.