I have a software recommendation for mixing MP3s - if you've not tried it then I strongly recommend the free app
Djay.
I'm not a DJ so I've only played with it - but it does seem pretty clever, it lets you mix music from your iTunes library and as said earlier - it's completely free!
It takes full advantage of Apple's latest audio and graphics technologies, and runs natively on both Intel and PowerPC processors. It supports all common file formats, such as mp3, aiff, wav, aac, caf. With djay you can control and mix your whole digital music collection with two realistic virtual, interactive turntables and both record and transmit the audio in real-time over the Bonjour network to other connected djays.
From Mac Music
EDIT:
The top features include:
• Real-time audio mix transmission over Bonjour network
• Real-time audio mix recording
• iTunes Integration
• Automatic artwork image loading on both turntables
• customizable vinyl
• Forward spinning
• Time-Stretching
• Pitch-Shifting with amazing blend effect
• 5 band Equalizer
• Audio Unit Effects Support
• Pre-Cueing (requires multichannel sound card)
• Crossfade automation
• Manual Beat Counting
• Magnetic tempo slider for precise beat matching
• Cue points for beat-juggling
• Full control with keyboard shortcuts
• GarageBand instruments integration via network
• AppleScriptability for mixing automation