I've tried calculations on the internet and stuff but I don't really trust them.
A friend o' mine wants to add another Radeon HD 4890 to his comp and he already has the following:
Case: Antec Ninehundred, with a 200mm fan and 5x 120mm fans
GFX Card: XFX Radeon HD 4890
Motherboard: ASUS P7P55D-E
RAM: OCZ Obsidian Series Triple Channel Kit 6GB (3x2) DDR3 1600MHz
CPU: Intel Core i7-860 @ Stock, might want it oc'd in some time
PSU: Coolmaster Real Power M620
HD: WD Caviar Black 1TB 7200 rpm 32 mb
Now, he wants to add another 4890 as mentioned earlier, but I'm not entirely sure if his PSU will hold it together as I have had bad experience with power supplies in the past (not mine, luckily) and I have no experience with Cooler Master PSU's either.
A quality 650W PSU will handle the rig fine. I just don't think CM Real power will be able to. A similar powered Corsair or Antec will be good enough. Go 750W just to be safe & sure. My money's on the 750TX too.