You might want to think about getting a program called ReCycle too.
It lets you take .wav loops and automatically cut them up so you can alter the bpm and add other effects. Then you save them as REX files.
If you use REX files in an audio program, you can adjust the bpm and it'll fit the beat to it.
I just grabbed some .wav files from PLP Loops and spent the last hour cutting em up in ReCycle. The major advantage is being able to take things like bass and synth loops, adjust the bpm as well as the pitch.
A lot of the loops were in Dm, so I saved several versions in different keys.
Then you can also save alternate versions like lo-fi, hard attack, low boost etc.
You can basically take one loop and use it in ten different songs that all sound different.