how strong a chess engine could be?

[mdtr]

i'm not talking about hardware, i'm wondering how much stronger (let's compare it with houdini 3) a chess engine could be (staying with the actual chess engine "paradigm", nothing too strange)
i'd go for +500 elo

edit: i'm asking about this because i remember i could not imagine an engine significantly stronger than rybka 2.3 haha, now with these super strong chess engines out i'm kind of believing that there is always a lot of things to add and tune to a chess engine