Realtemp shows wattage of just cpu, and seems to be reasonably accurate, as does DES chip on some GB mobos.

Just cpu wattage on my i7950 from DES chip, prime large ffts load, at 25C is:
HT on .......HT OFF
85W...........72W.......stock, 1.12V, 3.03GHZ.
136W........117W.......1.26V, 4.2ghz
180W........160W.......1.34V, 4.4ghz
255W........221W.......1.50v, 4.7ghz

And by comparison, only point i have on 2600K is HT on
HT on 99W, 1.27v, 4.5ghz, which implies about 30% lower power on 2600K vs i7950 for same settings.

Also, when I run my i950 outside at 0C temps, ie 25C temp drop, the power consumption on cpu drops about 20W at higher settings from reduced current leakage in cold, ie colder cpu the less power consumption, that is where it gets difficult to estimate power consumption in cold based on normal ambients.

I think 250-300W would be reasonable guess for max need for cooling. But if Realtemp shows wattage on your cpu, that would help you guess more than anything.