To answer your question though, the taxes would be the same as that much of an increase in income.
So if you make 50k, and you have a taxable conversion of 25k, you'll pay taxes April 2014 as if you made 75k.
If it is an old 401k that is currently not in service, there is no reason (and likely tax preferable) why you can't spread it out by partially converting over a few years.
(If I misread and you put it in a standard 401k, them yes, you have to pay taxes on it all, under the same rules as above.)
This post was edited on 2/20 at 10:42 am