Acidfix :
You need to look up you're PSU spec's. But it is not really about the wattage in the first place it is about the amp's. Also does you're PSU have two 6pin and two 8pin made for VGA?
I have a XFX XTR 750 and it is able to handle two 290's but you need to make sure you're PSU can too.
If you're PSU can't and does not have the amp's on the 12v rail to handle it, then I would say step up to a EVGA 1000 Watt PSU You will never need more power then that, at least for the next 5 year's.
I don't like to use two gpu's though. I run a single GPU and that's it.
Most people who SLI have more issue's then just being about to play the game. Crossfire is another story, with the new AMD GPU's you gain the extra power and they act as one GPU much better. But most people that have more then two GPU's in there system are benchmarkers and it's kinda weird imo. You sit and do benchmarks all day... I guess we do need people like that though so we know what card's are best to buy at the time. Right now AMD is on top and I have a feeling they will be for a long time to come. Mainly because the PS4 and XBox One both use AMD and AMD has come across something very good. I really don't buy a new GPU every year. It is more like every 2 or 3 year's depending on how much better the next gen card is. And with the 290X beating the 780ti for much less money. AMD is on the right path.
I do agree that it's not only the wattage you need to check but also the amperage. But there's another thing I would add is the quality of the PSU. Some manufacturer overrate their PSU while others underrate their PSU. I know Corsair most of time underrated their PSU. The Corsair 750TX is able to take 900w for 30 mins without a problem and I'm sure your XFX is a good quality one. Corsair underrated their TX and HX lines which is why they seem overpriced but they didn't underrated that much their CX lines as it cannot take a big overload for a long period and it's why they are cheaper for the same wattage.
Now when it comes to saying AMD is on top of Nvidia I can't agree even though I have a x1950 pro, 2 HD 3870x2, 2 HD 6950 (unlocked) and 2 R9 290 tri-x but sold a 8800gt and a GTX 480. If there is one point I do give to AMD is their performance/price ratio which is unbeatable in the high-end. But currently AMD consume more power for the same performance but this is mostly due to the fact Nvidia has a bigger chip so AMD has to clock their chips to a higher frequency for the same performance.
Bigger chip=more performance at the same frequency
Bigger chip=expensive
Bigger chip=more power consumption
Now let's take an example with rounds numbers so it's easier to understand.
If Nvidia make a chip with twice the transitors of AMD, it will be more than twice as expensive because it double the cost of production but also it increases the number of failures. But when they are produced, they will have near twice the performance at the same frequency for twice the power consumption.
Now AMD will most likely clock their chips to twice the frequency to have about the same performance but twice the frequency doesn't mean twice the power consumption, it means near 4 times the power consumption.
That explain why AMD chips are cheaper but consume more for the same performance (R9 290X vs GTX 780 TI) but Nvidia are way too expensive. Now you can the R9 290 and the GTX 780 which lower the cost of production since sometime they are chips with failures so they desactivate the units that failed. AMD aimed at a better performance/price ratio and Nvidia aimed at pure performance and better performance/power consumption. Personnaly I prefer AMD choice since I am only a student in physics and I'm poor
Crossfire and SLI can be useful if you want more performance but doesn't want a X2 card and most of the time, they have a better performance/price ratio. The reason whh I upgraded my main computer from 2 HD 6950 unlocked to 2 R9 290 tri-x is because I got them both for 550$ in may which was a killer deal. Some bitcoin farmer going from GPU farming to Nexus farming, something lime that he told me. And 2 R9 290 help getting 144 fps in all games.
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