I'm building a new computer this summer and I think I'm going to go for an Athlon 64 X2, so I'm interested in a socket AM2 motherboard that is both good quality and compatible with Linux. Recommendations on a specific wireless card that works nicely under madwifi would be appreciated as well.

I can't give you a specific model number, and this is the reason (and my warning):

Wireless card manufacturers don't always change the model number when they change chips; my Belkin FD7500 (I think that's the model) comes in Broadcom, Atheros, and Ralink models, and the only way to tell is looking very closely at the picture on the box! (or the card itself)

So the best thing to do, is figure out which Atheros chips work, get a card that you can see the little A on it on the picture on the box, and then save the receipt while you test it. Return if it doesn't work. Rinse and repeat.

Good luck.