Building a new desktop?

[llSTAYGOLDll]

Hello,
i'm building my new desktop using some newer components as well as using some of my current desktop components.
I'll be using my old thermal take case as well as my ATI 4870x2 in crossfire, heatmaster cooler and 2 HDD's & CD drive.
So basically just installing a newer processor, motherboard, ram and hopefully power supply.
I'm jobless at the moment and looking for work, so i will be looking to keep my budget under $400. (I already have a new Intel i7 - 950 LGA1366 from a year ago.)
For my motherboard i have picked out this asus LGA1366 Xeon: http://www.amazon.ca/Crossfirex-Workstation-Motherboard-P6X58-E-WS/dp/B004RPS010/ref=sr_1_2?ie=UTF8&qid=1354404323&sr=8-2

Ram: I heard 8 GB should usually do. http://www.newegg.ca/Product/Product.aspx?Item=N82E16820148543

Power supply i would like advice on, i'm not sure how much it would exactly take to power my i7, GFX, etc. So i just picked this one out, if possible please help me find a decent priced power supply capable.
http://www.newegg.ca/Product/Product.aspx?Item=N82E16817171031

Also, for a new motherboard, would i have to reinstall windows? My niece has lost my activation key and installation CD, so this might end up screwing everything up.

 

[rabidraccoon]

You probably want closer to ~600-700 Watts for that setup. 4 GB of RAM will be enough but it'll only save you like $30 so I'd go with 8 GB.

Use this to check how much power you need:

http://images10.newegg.com/BizIntell/tool/psucalc/index.html

A PSU for that wattage will cost $90-150. Here are some:

http://www.newegg.com/Product/Product.aspx?Item=N82E16817182067
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371025
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139010
http://www.newegg.com/Product/Product.aspx?Item=N82E16817182073

If you have OEM version of Windows, you'll need a new copy of the software for a new motherboard. Otherwise contact MS support, they'll give you a key and charge a fee (not sure how much it is.)

 

[o1die]

I would search your local craigslist or anandtech's for sale forums for a used 1366 board; you should find one for about half the new price, in case you need a new windows coa. That's also cheaper on craigslist. Ditch the sli factor if the price is right; those old video cards are too slow to spend extra for sli; you can get a newer single video card with the money saved and sell your old 4870's to help pay for it. If you insist on new, I would sell the old 950 cpu for around $125 and start over with an 1155 cpu/board.