Expresscards are a faster interface than PCMCIA, so yes they are here "to stay". They won't go back to PCMCIA... However, it wouldn't suprise me if 2-4 years from now a new cardslot is invented. Shoot, for all we know USB will be replaced by then.
X-Fi is the top of the line sound offered by creative. It has a dedicated processor inside that only handles sound. This removes some pressure off of the CPU, although probably not all that much. If you have a Core 2 Duo 2.00 ghz, you won't see any speed increase I don't think. The audigy 2 is fairly outdated, as the audigy 4 was used for a while and now the X-Fi. Its similar to comparing a Geforce 6600 to a Geforce 8600.
I don't understand how it all works underneath really... I thought the X-Fi was the only 24-bit sound card out there, but after looking up a quick Audigy 2 search, it appears that some of those were 24-bit also. All I really know is, I used to have a Sound Blaster, then a Sound Blaster 64, then Sound Blaster Live, then Sound Blaster Audigy 2, then Sound Blaster X-Fi... It keeps improving.
If you have $10 earbuds you bought at Wal-mart, you don't need this card. If you have $450 headphones, you need this card. If you want to hook the laptop to surround sound, you need this card. If you want to hook the laptop to $20 2.1 speakers from Fred Meyer, you don't need this card. If you use the expresscard slot for your WWAN card but need a sound upgrade, get USB. If you don't use that slot and don't want a bulky USB thingamajig to carry around (like me) you'll prefer this item.