It depends on what you intend to do with REW and also what other hardware you have or intend to use. The Creative Soundblaster units are “old school” with REW and can only be used along with an SPL meter, whose internal mic is used for the measurements. This is a decent and inexpensive method for subwoofer-only measurements, especially if you already own a meter, but is not accurate for full-range measurements.
A significantly upgraded option is to use a mic suitable for full-range measurement along with a soundcard that has mic pre-amps and phantom power (to power the mic - the Soundblaster soundcards have neither of these). You can find recommendations for these interfaces (as they are commonly called) in the REW Soundcard Database. A further upgrade would be to use a mic with custom calibration from Cross Spectrum Labs. With the calibrated mic and suitable interface you have a first-class rig that can do just about anything you want, room-measurement-wise, all for about $200.
However if you’re starting with no hardware at all, these days it makes the most sense to get a USB mic such as the UMIK-1 or UMM-6, which can be had from Cross Spectrum for about half the price of the mic w/interface option. These mics can be had cheaper from miniDSP and Parts Express respectively, but CSL’s calibration is more accurate (especially in the case of the UMM-6). With USB mics no other hardware is needed; the computer itself becomes the soundcard. You can utilize either its headphone or HDMI output.