so in my days of internetting, i've come acrosse several really sweet mashups, usually combining a new-age song with a classic (e.g. tupac rapping over the beatles)

i was wondering how to go about creating a mash-up, as i think that it would be a really cool and fun project. Obviously you would have to find two songs with the same BPM and all, but how can you know the exact BPM of a song?

and does anyone know of any software or programs that would help me in my epic quest?

thanks in advance, any help would be greatly appreciated.
if you're going to flame, go home.
I have a mixtape that is Notorious BIG rapping over Jimi Hendrix songs, a few are modified, but it's still cool. Not great quality though.
If morning's a bitch with open arms, night's a girl who's gone too far.

there's videos on your tube of a guy doing metal drumming over a few popular club rap songs"