It really all depends on your budget and what you want to do:
Do you want multi bitrate streaming?
Who is your content delivery network? Do they do transcoding for multi bitrate?
Do you have a fast enough computer to handle streaming (quad core i7 or similar )? Then some of the black magic design cards that work well. Windows seems to do better with Flash Media Live Encoder and most of the black magic design cards. I can vouch for the BMD Intensity with a Mac mini server and wirecast for 720p + 360p multi bitrate streaming with Streamhoster.com (great support)
If you have some money, you can get a hardware based encoder that does rtmp streaming
One that works only on livestream only ($495+$45 month):
http://store.livestream.com/products/li ... roadcasterOne that does multi bit rate for any flash based cdn (thousands+cdn cost):
http://www.winkstreaming.com/en/wink_encoder/