Will 700 watts be enough to crossfire these two graphics cards?

Pollin

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I have a Radeon HD 7950, and I'm looking to upgrade to an R9 290. However, I'd like to keep the HD 7950 as a backup, and occasionally crossfire them. I don't even know if this will give me more power but if it would, then would a 700 watt PSU be enough? One other thing I should mention is that both slots on my motherboard are PCIe 2.0 (so there'll be a slight performance loss) and while both slots are for a GPU, one only run at x4 speeds, while the other runs at x16. I would move the 7950 to the x4 slot and put the R9 290 in the x16 slot.
 
Solution
You can't crossfire those cards. You could crossfire a R9 280 and a 7950, but not a 290. As for the performance loss due to PCI-e 2.0, there wont be, not from that anyways, running in a x4 slot would though. The current generation of cards run the same on 2.0 and 3.0 if the lanes are equal (x8, x8 or x16, x16)

If you want to crossfire the 290, you'll have to get 2 of them. The 290 is not a rebadge, it and the 290X are new cards. Rebadges started with the 280X, which is a slightly underclocked 7970ghz edition.
No, I don't think your plan will work.

Can you actually crossfire the two different cards? I have not looked into that.
even if you could, running one at X4 will be a negative.
And dual cards of slightly different capabilities will lead to tearing and stuttering.
Probably not worth any fps increase.

Past that, a R9-290 typically wants a 650w psu:
http://www.realhardtechx.com/index_archivos/Page362.htm
Add in a 7950 which needs two 75w connectors and the pcie slot power of another 75w and the added 225w is likely to give your psu a headache.

Then, your 7950 is a decent card, and I wonder how much better a R9-290 would be.
You may be disappointed if you don't see big results.

If budget were no issue, buy a GTX980 and sell your 7950.
 
You can't crossfire those cards. You could crossfire a R9 280 and a 7950, but not a 290. As for the performance loss due to PCI-e 2.0, there wont be, not from that anyways, running in a x4 slot would though. The current generation of cards run the same on 2.0 and 3.0 if the lanes are equal (x8, x8 or x16, x16)

If you want to crossfire the 290, you'll have to get 2 of them. The 290 is not a rebadge, it and the 290X are new cards. Rebadges started with the 280X, which is a slightly underclocked 7970ghz edition.
 
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Pollin

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What about two 7950s? Would that still give me a bit of performance boost?
 
I think not with a x4 slot for the second card.

What is the rest of your parts?
Are you certain that your gpu is holding you back?

To help clarify your CPU/GPU options, run these two tests:

a) Run your games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely more cpu limited.

b) Limit your cpu, either by reducing the OC, or, in windows power management, limit the maximum cpu% to something like 70%.
Go to control panel/power options/change plan settings/change advanced power settings/processor power management/maximum processor state/
This will simulate what a lack of cpu power will do.
Conversely what a 30% improvement in core speed might do.

You could also experiment with removing one core in the bios. You can also do this in the windows start configuration.
This will tell you how sensitive your games are to the benefits of many cores.

If your FPS drops significantly, it is an indicator that your cpu is the limiting factor, and a cpu upgrade is in order.

It is possible that both tests are positive, indicating that you have a well balanced system, and both cpu and gpu need to be upgraded to get better gaming FPS.