Depends a little on the audio format you are using, but at 320MP3, you are looking at about 2,5 (actually 2,34 or so) MB per minute. So let’s say 4 minutes a track gives you 10 MB per track. Which is 100 per GB. A simple 8 GB stick will give you 800 tracks. Let’s also assume that you’ll play (on average) 3 minutes of any track (the rest is lost in mixing and transitioning). That means you need 20 tracks per hour. For a 4 hour set you need twice the amount of music as can fit into that period (you want to be able to manoeuvre after all). 4 hours times 20 tracks times 2 = 160 tracks.
Conclusion 1: For a 4 hour set, you could do easily with a 4 GB stick (160 tracks times 10 MB = 1,6GB).
The biggest pitfall for digital DJs is having TOO much music in their collection. I think a decent size for your primary/main collection is somewhere between 1.000 and 2.000 tracks max (just my opinion), as this is the maximum amount of music you can know intimately enough to use.
Conclusion 2: if you want to take your entire core collection with you to a gig, a 2000 track collection would need approximately 2000 time 10 = 20GB. Some software will take up some extra space (like waveform info and such), but a 32GB stick will be plenty.
Hope that helps.
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