Computer turns off when gaming with crossfire

Hello
Can anyone guess at an explanation for a problem I am having? When I play certain games with crossfire enabled, my computer will just decide to turn off. When I play the same game with crossfire disabled, it will play fine (but obviously without the performance boost). For example, I just started the new prince of persia and when I have crossfire on, I can get through the menus and the first cut scene but when the first in game cut scene starts, my computer turns off. The same thing happened with Mass Effect a while back.
Following are my specs. I appreciate any help anyone can offer. Thanks.
AMD 64X2 4400 2.21GHz
4gb ram
Vista 64bit ultimate
2 sapphire x3870's (single slot)
4 250gb hard drives in raid 0
4 case fans, neon lights
Dell 30" monitor (2560x1600)
Asus A8R-MVP32 Deluxe motherboard
Creative x-fi xtrememusic soundcard
550watt psu
I don't think it is a hardware problem with the cards because I have successfully played some games in crossfire before. I don't know how many watts this setup is so I wonder about the power supply. It was sli certified but I wonder if it might still be too small. Also, I should note that nothing is overclocked. Thanks for any help.
Brandon

Agreed.... PSU POO POO

750W or more is too much, 650W will suffice.

Be careful of the PSU you buy. I had this EXACT same problem and I was running a 700 watt PSU. At first I thought it was a heat problem but after I ruled that out I ran ASUS probe and kept the voltage monitor active started crysis. Gaming with Xfire draws alot of power and even though my PSU was 700 watts the 12v rail couldnt handle my 4850's running in Xfire. As soon as things started getting busy in the rail the power on the 12v railed dropped to 11 then 10 then the computer cut off. The PSU overheated and went into thermal protection, had to reset the switch on the actual PSU and let it cool off before the system would cut back on. The PSU was fine running everything including 2 4850's with 3 monitors as long as I didnt enable crossfire and start pushing the cards.

Make sure to get a quality PSU and if possible go for one that is ATI certified. I replaced my 700 watt with a Toughpower 750 watt that is certified and haven't had a problem since. Its not all about the wattage the 50 extra watts wasnt the difference, the psu has to be quality. For crossfire on your cards I beleive ATI recommends 600 watts minimum but going up higher to around 750 wouldnt be a bad idea with all you're running in your system.

I would tend to stick with something like PC Power and Cooling or Corsair. Seasonic is good as well.

Also, make sure you go for something that has plenty of amperage on the 12v rail. Around 45-50A is a good idea (it doesn't matter whether it's a single rail, or several smaller ones that add up to 45-50A, as long as it has the power). The Corsair TX650W or TX750W are fairly inexpensive, and great quality for example. They both have >50A on the 12v, and are excellent choices.

I would never buy an Ultra PSU. I think that's the most common brand I see of ones that blow up.

+1 cjl - PCP&C, Corsair, Seasonic. I would add Thermaltake and Antec to the list, personally.

Ok, what about this one: OCZ StealthXtreme 700w SLI (120mm, 2 PCI-E , PFC,4-12v rails, ATX2.2) *** ($35MIR = $64)? It is only 700w but it has 4-12v rails and is name brand.
I basically want to get the cheapest one that will do the job from my local PC store (I trust them and I want to get the psu installed for me, I'm too lazy to tie all the wires up nicely like I know they will). Here is a link to their inventory in case someone thinks that they have a better psu option in the $100ish pricerange. http://www.otvtech.com/pricelist/otvpricelist.pdf
Thanks again for the responses.

Out of whats on the list the one you picked seems to be a descent choice and the best bargain. Not a ATI certified but the fact that its powerful enough for SLI means it should handle running your two cards just fine... If you gotta go with one from that store then this seems like the way to go.

The one I wanted was out of stock so I ended up getting OCZ ModXStream Pro 700w SLI (120mm, 2 PCI-E , PFC,4-12v rails, ATX2.2) *** ($35MIR = $85). It is only about $100cdn after rebate. I installed it myself (bit of a pain) and it seems to have done the trick. I am now playing Prince of Persia in stunning 2560x1600 with crossfire.
Thanks to everyone who replied with advice. You all rock!