Really depends what you want, Recently put together a system for a friend.
i7 860, 4Gb ram, Nvidia 275GTX, etc etc...
The whole thing only draws 69W once windows is booted up, around 75W while web browsing, its pretty efficent. Once you start gaming its drawing around 170W or so, but really for a machine that can play the majority of titles with a decent level of eye candy, its a remarkably power efficient system.
Compared to the previous generation intel chips i7's are very power efficient when lightly loaded as they can drop to a much lower clock speed, and also they can turn off unused cores completely.
As already said in this thread, the size of the PSU doesnt matter, just the efficiency. Putting a 1k PSU on a system thats drawing 69W idle is going to decrease power efficiency, likewise a 300W psu on a system drawing 275W isnt a good idea either. Best to keep the PSU running between 30 and 70% for both efficiency, and long life for the PSU.
AFAIK the most efficient current PSU is the latest modular PSU from Seasonic, Unfortunatly OcUK dont have stock at the moment (or have given up selling seasonic)