1) Wireless cards don't crack encryption, WEP or otherwise. Programs crack encryption. Cards transmit and receive data, and may or may not work in different modes (e.g. monitor) depending on on what drivers that are available for them under a give OS. Data captured from a card is then fed to a cracking program.
2) Cracking WEP is a simple, and IMHO, a very lame, attack. It's old, it's easy to do, and WEP is known to be vulnerable, so what's the point in doing it? These days, there are only two valid reasons to crack WEP encryption:
a) You're a professional and a client that requires that you prove it's possible.
b) You want to actually try it once, just to say you've done it.
The only other reason isn't valid, it's just that you're a low-life thief, and want to steal access off some hapless victim.