I have an xbox 360 and i used to stream from my linkstaion pro nas using twonky media as a upnp server and i worked pretty well, the 360 plays .avi .mp4. .mp3 .divx so not too bad. I sold my NAS recently and built a media server, i'm still using my 360 to stream but will be building a custom built mini itx htpc and putting XMBC on it. Download XMBC live cd and give it a try, it's fantastic, it looks amazing and plays a crap load of stuff.
I like the idea of the acer revo as it's cheap at £150 and is more upgradable than apple tv which is under powered and over priced.