The audio interface that I use is a budget audio interface. Tascam US-200 24-bit Audio [2-in (one combo / one xlr), 4-out (rca unbalanced)] / Midi [1-in, 1-out] USB 2.0 interface.
It's the cheapest 24-bit USB 2.0 interface on the market. It's pretty solid. The only thing I don't like about it is the out puts are RCA / unbalanced (would prefer TRS balanced 1/4 outputs... but it's less than $200 so I guess I can't complain). The only other problem I had with it is I tried to do a midi dump onto my hardware synth (uploading some system executable files aka sysex files) and apparently the midi protocol for the Tascam US-200 isn't as compliant as specialized USB/MIDI interfaces (thankfully I didn't get rid of my M-audio 2x2 midisport). Aside from that it does midi control in-put and out-put just fine. No problems anywhere else. I got a price matched from zzounds for around $138. For sure the very cheapest USB 2.0 24-bit audio interface you will find on the market (considering even most "prosumer" level M-audio audio interfaces... if they are 24-bit they might be USB 1.1... Though they might have a nice built-in preamp and possibly balenced outputs, I would rather have USB 2.0 to minimize latency). Though I would prefer an IEEE1349A (aka firewire 400) interface... but firewire is complicated and if you don't have the correct chipset (TI IEEE1349A chipset of some sort) on your motherboad then it won't work as well... Plus Firewire is just more expensive... If you have the money... Firewire is the way to go (though people might argue that USB 2.0 is faster than firewire 400, the burst rate is faster yes, but the constant rate is notably faster making firewire 400 and obviously 800 better for real-time processing). But if you are poor like me, then look no further than the Tascam US-200 audio/midi interface. If you go that route, I recommend spending $50 dollars and getting the Ebtech hum X as the ground wire on your laptop creates static interferences that bleeds through to your audio signal... The only other way of "lifting" that interference is chopping off your groundwire (not recommended), using a 3-2 plug adapter (cheap... I did that for a while.. cost three bucks... but my battery on my laptop started messing up due to lack of ground) or going on battery power (if you have a nice 12-cell battery on your laptop, then you might last for an hour or more but if you have bus-powered USB devices you won't make it through a set on battery).
Lastly the 4-out on this thing is nice because you can use it with a 2/3 channel external DJ mixer or you can use 2 for your main out and 2 for headphone mode in mixxx... However, I have had difficulty with the secondary channel messing up on headphone mode... I'm not sure if it's because of mixxx or the interface... But the main out is solid. I don't think it's the interface itself because I have used mixxx on one channel and a seperate VST on the other many times without any issues.