The AppleTV will do it, but it's not that simple.
If you already have a library that consists of mostly iTunes material in .m4v or .aac format, then it'll work great. However, it sounds like you have files that are mostly incompatible with the AppleTV, in which case you'll need to remux and/or reencode your files to make them work.
Boxee Box is pretty good as well as the Dune and Popcorn hour players. Pivos Aios makes a box that is inexpensive and plays about everything but the UI is terribly user unfriendly. Same with the Micca boxes. Asus makes a couple that I haven't seen in real life and the Netgear boxes support Cinavia so they would be just about the last on my list.
But, if you haven't made up your mind about an AppleTV, yet, take a look at the Western Digital TV Live or Live Hub. There simply is nothing better for ther $ for playback of locally stored media. They support just about every media format and have a pretty easy UI. They have quite a few online services, but not to the extent of Roku.
You may also want to consider a mid-priced Panasonic, Samsung or LG Blu-Ray player but, once again, you'll have the Cinavia issue with those, too.
As far as wired vs. wireless... I used to stream HD across a 5Ghz, 300mbps, N connection and can honestly say it worked pretty well as long as the signal was pretty strong. I wouldn't try it on anything less. Wired is always better for this, if possible. Even a 100mbps wired is gonna perform better than any wireless N connection.