[Burt]

As far as I can tell, looking at the star trek Encyclopedia, Impulse speed is is around one-quarter the speed of light. Although this is I think the norm for around the Next Gen era, so back then, in Enterprise, Impulse might be even slower.
There's a niffy little chart in the Book to, that explains the differt speeds too.

At warp Nine, Five Light years takes - 1 Day
At warp Five it takes - 9 Days
At Impuse - 20 Years.

One point that has yet to be explained (to me at least) is the difference between the 'old' warp speeds and the 'new' warp speeds, and there relevence in Enterprise.
We know that back in Kirks time the Enterprise reached warp speeds of around warp 14. But when Next Gen started, old Gene wanted to put warp 10 at the top. So all the older speeds have been 'recalibrated' - meaning they were less than warp 10. Kirk's warp 11.8 is Janeway's warp 9.2. Ok. Heres the bit:
Enterprise has a top speed of warp 5. Would that mean they are really going around warp 2-3?