I always wonder with these boards why wifi isn't just built-in. It seems like a delightfully simpler interface than any wire, and means the base system can integrate with anything else over the network. Sure you can always add wifi, but the point of these boards is that they bundle all the basic stuff, and often include things like development environments that can't make use of wifi if they don't know if it is avail…
If you need WiFi, you could start with one of those low-cost DD-WRT/OpenWrt compatible routers ($15-20 or so: TP-Link TL-WR740N http://wiki.openwrt.org/toh/tp-link/tl-wr740n). They have little RAM, but plenty of I/O (serial, RJ45) and they're still almost "palm-size".