Riff, there are several routes you can take. Here are some bits from personal experience:
-if you have no turntables but just a laptop, you can use Native Instruments' Traktor DJ and hook up a fader MIDI controller to toggle between your two virtual turntables on the desktop of laptop. The drawback is, you can't beatmatch the tracks as you would pieces of vinyl - I prefer to have something to handle since I learned mixing records the old school way.
-if you want to remix live, you can use Ableton live, a very popular DJ tool that also doubles as a music writing tool capable of recording and sequencing. I use this along with a Korg Kontrol 49 in the studio to trigger "scenes" and loops for live remixing. I use this a lot and it never leaves my laptop since it allows me to work on songs when I am flying or get stuck at airports in between flights. Along with Reason 3, this combo keeps me occupied and happy while the other passengers pace restlessly waiting for the plane to load:)
-if you have existing tables and want to integrate the use of mp3s or some kind of digital media, you can use Final Scratch which uses encoded pieces of "vinyl" to play your mp3s on conventional turntables. An interface comes with this hardware solution that you hookup to your mixer.
Here is an example from my own setup so you have an idea and can modify it as per your needs. The reason why I have Final Scratch is to use it as a way of playing remixes without pressing any CDs or pieces of vinyl. It helps to know if a mix can get a dancefloor going before submitting it to a pressing plant for records to be made.
Technics SL 1200 MK3 - Rane Empath Mixer Channel 1 Phono In
Technics Sl 1200 MK3 - Mixer Channel 2 Phono In
Final Scratch Vinyl (from Laptop Output) - Mixer Channel 3 Phono In
Booth Monitor Outs - Event PS 6 monitors
Main Out - Audio Interface to Computer
