Both systems are 5.1, not 2.1. In any event, I would prefer the X530 - While both offer nearly identical power output, the 530 is a newer model with better features. Looks better too IMO.
640 Specs:
# Experience Dolby Digital sound for music, movies and games
# Innovative, dual-chamber subwoofer delivers twice the bass energy of similar subwoofers
# Four stylish satellites stand on desk or mount on wall to provide smooth mid and high frequencies
# Center channel speaker with system controls rests on your monitor or your desk
# Magnetic shielding in speakers protects your equipment
# Internal power supply eliminates bulky wall adapters
530 specs:
* Frequency Directed Dual Driver (FDD2) satellite technology produces a uniform sound field for full, rich sound
* Matching front & rear satellites deliver balanced surround sound
* Dedicated center channel makes dialogue clear and intelligible
* Dynamic, real-time bass equalization maximizes bass response while minimizing distortion
* Ported subwoofer design displaces more air for deeper bass
* Headphone jack enables private listening
* Master volume and subwoofer level controls placed for quick, convenient access
* Brilliant design allows speaker to be used on table top or wall mounted
* Booming, crisp audio at amazing 70 watts RMS
* Wide frequency response of 40 Hz - 20 kHz