Building a Computer and need some suggestions

[Phantom0981]

So recently I have been having issues with my computer which some of you helped me solve...well I thought it was solved. I came to find out that it was only a temporary solution and my computer has slowed back down. I have been needing a new desktop anyways (4 years old) and I thought that now was as good of time as any to purchase a new one. So I have decided to finally build a computer on my own and donate this computer to my brother (should work for him since he is not a big computer person) since it still is running. I have been looking over parts and I have found everything that I would like but I have a question on the motherboard I have selected as well as the graphics cards.

Motherboard:

EVGA X58 SLI Classified Motherboard (Model: 141-BL-E760-A1)

Graphics Card:

(2) XFX Radeon HD 5850 Black Edition Video Card (Model: HD585XZABC)
* Will buy 2 so that I can Crossfire

So here is my question:

I am really unsure which is better, ATI or Nvidia? My friends say that ATI is much better than Nvidia recently and I was wondering if that is correct (Explanations as to why you prefer one over the other please). Also the motherboard is made by EVGA (Nvidia I believe) and it says that it is SLI ready and Crossfire Ready (Though only on some websites). I would assume that since it is SLI ready it would work with Crossfire as well and it has plenty of PCI Express slots. Any help would be appreciated.

Also, I going to buy a 900 watt power supply and was wondering if this would be overkill or just enough. Here are the rest of the specs if you need them.

12 GB (6 x 2 GB) Corsair Dominator PC 12800 RAM
1 CD/DVD Drive
2 1.5 TB HDD (RAID 1)
Intel i7 930 Processor
ULTRA m998 Case

Any suggestions on any of the other parts I have selected is welcome.

Thanks in advance!!

[Bones]

I had Nvidia Geforce GTX275 Super OverClocked for 5 weeks and it was good until it's GPU got blown  :eek yeah with 5 fans venting whole PC I couldn't cool that thing down, it was overheating all the time... soon BSOD kicked in and then artifacts and then .... scrap yard I admit it was powerful but come on, no DX11 support, loud as hell, hotter than hell, temperature leads it to it's own death - crap!

Now I'm happy user of XFX Radeon HD 5770 and it's complete opposite to GTX quiet, keeps low temp, even more powerful, has DX11 support, much better than Nvidia ;)

[ECGadget]

On my PC I've got a (dated) NVidia GeForce 8800GTS at 512mb... its really good. However, on the laptop I just bought has a Radeon HD6560 on it at 1gb... its a sweeeet card

[Kirk]

Make sure you do extensive research to ensure your different components will work together.

[Phantom0981]

Make sure you do extensive research to ensure your different components will work together.

Ok I believe I have researched it pretty well. The mother board does DDR3 RAM up to 1600Mhz and The Corsair is DDR3 1600Mhz so I'm assuming they will work together.
The i7 processor has a LGA 1366 Socket as does the motherboard
The HDDs are connected via SATA cables I believe and the motherboard has those connectors and supports RAID 0,1,5,10
The graphics cards are PCI Ex 2.0 and the motherboard has 3 PCI Ex 2.0 card slots.
The DVD/CD Combo is also connected via SATA as well.

I believe that all of the components will work together but if there is something I am missing that I didn't cover let me know. I know quite a bit about computers but I have never built one before. Always went with HP.

Thanks!!

And looks like I'll stick with the ATI graphics cards.

[milenent]

Why are you going to do RAID1. And here is another one. Asus M4A88M-T or ASROCK 790GX PRO?

[Meteorafallen]

From what I have been reading, AMD (Yeah their cards are just AMD now, ATI is no more) are currently the more advanced cards when it comes to DX11, with Nvidia quickly catching up. Your power supply may not be enough, with most higher end cards requiring a 400 watt power supply you may be pushing it. And proper cooling is a must, if you have the funds I would suggest a water cooled option for that processor.