Asus M3A78-EM here with AMD Athlon X2 5000 (Purchased about a year ago on Newegg and I think I left a review there too). I've had Debian Etch, Lenny and Squeeze on this machine with no problems. The gigabit ethernet works fine. As far as speed goes, I wouldn't call it "badass" or anything, but it's certainly not slow (but I'm not a gamer, I don't overclock, test the cpu, etc).
My Linux distro of choice is stable Debian=Lenny 5.02. However, over the years I've learned that just because one Linux distro doesn't work does not mean another won't. Trying different distros will often reveal clues as to why another may not have worked. (I'm no geek, and these things are often hit and miss for me). Anyway, as I recall, I tried Ubuntu and I think it installed but there was a video problem. Fedora and sidux worked. Zenwalk worked and was faster than anything I've ever used. (Much faster than Debian).
My next mobo I am going to try a Gigabyte to compare to the Asus.