Are you planning on using the DTXpress as a MIDI trigger for a drum module on your computer (like Addictive Drums), or using the stock sounds from the DTXpress straight into your DAW?
A simple audio interface with MIDI in/out will suffice, just in case you want to try both methods, like the M-Audio Fast Track Pro. I'd get a two-channel interface to record things in stereo.
To use it as a MIDI trigger you'll also need a MIDI cable to connect the MIDI out of the DTXpress to the MIDI in of the interface, then load the correct drum map on the drum module in your DAW (Addicted comes with the correct drum map for Yamaha electronic drums) otherwise, you'll have to map it yourself. Then there's the slightly troublesome part of configuring the drum module to record on a MIDI track. This method is really useful for post editing, like quantizing or editing MIDI notes to have different sounds with the same performance.
The other method is more straight-forward. Plug a 1/4" cable out of the mono out of the DTXpress (or two if you want stereo, highly recommended for drum mixes) into the audio interface, and just record it like you would any other line-level instrument. I haven't used the DTXpress IV before, I have a DTXplorer and I have to do lots of tricks on it post-processing to sound decent. Maybe your tones are a lot more decent sounding than mine!