For any of the above mentioned upgrades that involve crossfire, you need a bigger power supply. Period.
Here are some links to power usage for your hardware. These numbers are all stock performance, no overclock:
Graphics cards: http://www.legitreviews.com/article/1488/18/
Q9550+6950Xfire = 187W+479W = 666W
Q9550+6970Xfire = 187W+596W = 783W
Q9550+6970single = 187W+348W = 535W
Tack on an additional 70W to 100W if you decided to OC the Q9550; those Xfire numbers are above your power supply's best case scenario.
All of that above would be assuming your power supply had a single 12V rail to feed the entire 750W. You don't though, your power supply only has 720W combined on four (4) different 12V rails
. So you'll never get that full potential out of the power supply. Also don't forget that there are other things in your computer that require power also, like the motherboard, RAM, HDD, etc... They may not require a lot, but you still need to account for it.
If you don't get a different power supply, do you really want to push your current one that hard? That'll be a lot of expensive hardware to replace if your power supply blows out the 12v rails when it eventually fails.Edited by bane-o - 11/5/11 at 9:54am