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If you are getting an HD5670 I would get the GDDR5 version for a few quid more:
http://www.overclockers.co.uk/showproduct.php?prodid=GX-120-HT&groupid=701&catid=56&subcat=1673
TBH, an HD5670 GDDR3 should be called an overclocked HD5570!
That ones only 512 mb though.
The loss of memory bandwidth makes more of a difference than the amount of RAM on an HD5670.