Well, they're bound to vary to some extent in terms of quality and reliability, as with most things. Also open to variation is the connection protocol; most cards are for standard 802.11g plus the older 11a, but some cards are dedicated to certain "pre-n" standards, such as MIMO. Multi-in Multi-out works be splitting the signal across three or more aerials and bouncing each segment off surfaces. Other "pre-n" standards - and I use that word very loosely! - are boosted in some way. The forthcoming - a while away yet - 802.11n will run in the region of 108Mbits per sec, and the "pre-n" systems - which cannot be mixed - are designed to mimic this. 11g works at 54Mbits per sec.