I use a 5V 1.5A power supply (thats a max output of 7.5W) and it powers my raspberry pi with ethernet cable, as well as 750mA 2.5" HDD its using to improve throughput.
I can download/upload to the Pi around 20MB/s this way, opposed to around 4-5MB/s off SD, plus I have a TB of storage.
Highly suggest anyone who wants real throughput from the Pi follow this guide: http://zeroset.wordp...ash-drive-stick
My XBMC box runs from a 5V 1A supply without any problems, and if you use HDMI-CEC and have no need for a wifi adapter you can likely get away with a 5V 0.5A or 0.75A supply. Ensure you are using a stable quality supply and a good microUSB power cable. 90% of raspberry pi issues come from these 2 things.
Most routers use 24W (12V 2A) you can determine this by unplugging the AC adapter and reading it. You should see something labeled OUTPUT: 12V --- 05.A (should be a solid bar with dashes under it) this states it can supply 12Volts at 0.5Amps.
Power is Voltage multiplied by Amps, so just multiply them together and thats the MAX amount of power it can use under full load. In the case above, 12*0.5, it'd be 6W of power. Most devices operate at most in the 60-80% of their supplies rating, to prevent failure with power brownouts or spikes. For maths easy we assume it uses the MAX power all the time.
You can do a lot of stuff on them too. like running XBMC while downloading torrents and having an SSH VPN tunnel is just the tip of the iceberg.