It also depends upon the intended audience. If it will be viewed on a standard TV, the best you can hope for is around 560x420 (a bit more if there are no borders or margins in the viewer). If it will be viewed on an HDTV, you jumpt to around 850x480, and for plasma screens it’s upwards of 1024x768.
http://developer.msntv.com
http://www.plasmatvbuyingguide.com/plasmatvreviews/resolution-explained.html