i have choosen ati readon 7770 ,was confused in 6850 also,but 7770 was latest so i choosed 7770 hd....but cant decide its motherboard bcz 7770 has 3.0 pci slot, i cant find motherboard with 3.0.....also my processor is amd 4100 and 4 gb ram......also what is crossfire and how it is done......which to buy 7770 or 7770 oc.....?
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Get any Mobo that fits your Processor.
PCI slot doesn't matters much, but that Mobo should have one.
7770 would be good. You can Overclock it yourself.
Yes all PCI-e Standards are backwards compatible so a 3.0 card will work perfectly in a 2.0 and 1.0 slot.
Also buy the regular 7770. Most of them can be easily overclocked to 1200mhz!