Hmm thats an interesting question. I'd say wireless would be a bad idea unless it was 802.11a. That has a max speed of 54 Mbits, which is good, but remember that will drop as you move the laptop around. I don't know if at the edges of your coverage whether a 802.11b setup would be good enough. If you have a 10 Mbit ethernet lan that will be good enough for video streaming, but since going 100 Mbits is barely more try that.
As for laptop speed, for playing DivX i'd say you want at the least 500 mhz? Thats minimum though, I'd want more CPU speed to play it better. Mpegs can usually run on systems with less than 500, but in all reality do you want anything less than 500 mhz? For a laptop that's my floor speed.