I don't know.... Streaming is VERY CPU dependent... my friend with a Q6600 couldn't stream sc2/lol/dota that well but after he switched to a i7-920, he can stream everything fine...
I guess it depends on the resolution of the stream the op is doing. if he's trying to do 1080p streaming, there's no chance in hell he can do it. but if he does 640x480, it might be fine.
I though this is where the i3's HT would be effective, but yeah your right if you already have the cpu give it a go, if not then looks like you should get an i5.
But then again it is dependent on your other component's.