Is it possible to crossfire sapphire 4850 1.00 GB with 512 MB

Yes, but check to make sure you have enough amps on your 12V rail(s).

Also one more thing to remember about Crossfire, it doesn't combine the memory for both the cards. It only has the two GPU's working together. The memory will be set by which card is used as the master card (one with your monitor connected to it). So make sure your 1Gb card is the one you use as the master(usually the card in the top slot) that way its not using the memory on the card with 512mb.

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it might work give it a try.

Crossfiring the cards will be no problem. I'm not sure on the memory thing but even if the clock speeds werent the same, it would still work, just the faster clock would be slowed down to match the slower clock speed. You could also overclock the slower GPU. In fact, you can even cross fire a 4850 with a 4870 although it is not recommended. For the power supply, I personally would avoid single 12V rails. I run an OCZ 700 watt PS with 4 12v rails.
http://www.newegg.com/Product/Prod [...] 6817341002

Personal preference. You can't say single is better then multiple or vice versa. Just when buying multiple rails you need to make sure that the m/b, PCI-E1 and PCI-E2 are all on different rails. Most rails are rated at 18A which is plenty of power to run anything you would have on that rail, plus even though it's rated at 18A, many can handle up to 20A.