ooh oooh, i found the exact video you need to solve the specific problem you asked for, AgiKvr:

http://www.youtube.com/watch?v=c13UP4lrYkg&feature=BFa&list= SP43D4D4C9D4213E3C

Go to 4:00 and watch to the end of the video. It does exactly what you want.

nix808. I was a diehard like you, unwilling to consider that Acid was decrepit, neglected laggard that was so far behind every other DAW it was holding me back. I was using Acid Pro 7 3 years ago and it hasn't improved simce then. My problem was that the only thing Acid does ok is making collages of loops and I was writing songs that required MIDI, VSTis, live instruments, etc. (It's not even good at making original loops, so pretty much all people do with it is stitch together loops other people produced.) I researched every DAW on the market and realized Acid was so far behind it was laughable. I had been wasting so much time. My music production skills have come lightyears. If you're just splicing loop libraries together, then maybe Acid is good enough. But if you want to actually compose music, work with live musicians, work with most of the newest VSTs and VSTis, forget it.

It sounds like I'm being mean, but I'm not trying to be I hope this will spur you to look at other DAWs--try Cubase or Studio One, for example--and you will PM me with a giant thanks

hmm...
I kinda like Cubase,
mmm high technology!hehe
The DAWS I have atm are EnergyXT,Acid,Usine,Orion,MAX/msp
Fruity,Reason.
I use them for different things,
so if there was an uber DAW that did all of these things
to my taste, I'd be in.
Acid is an option IMO,
but you have to do the boring job of cutting a take
into loops.
I use Acid to sequence loops I have recorded, and quantize them.
It gives machine timing, but like I say, it's boring--
plus u can make mistakes which throw a spanner in the works.
It didn't sound mean,
thanks for intending my capability

Thanx guys for ALL your inputs
as soon as possible I'll watch all those videos

I'd be hesitant to take a good song and stretch it (or parts of it) just to produce over it, lock it to a grid, as opposed to just mapping the tempo and leaving the audio alone to do much the same... unless of course, changing the tempo is actually part of the remix or whatever.

Melodyne does tempo detection but I haven't used it much so I can't say how effective it may be for freewheeling material. Some hosts will use transient detection (like 1/4 note transients from drums) and make a tempo map out of that so that might be an option to avoid chopping or stretching the audio.

I just don't much like time stretching good mixes unless it's really necessary.