If you are not going to change a BIOS, you are denying yourself quite much of performance boost. With factory BIOS it will be 22Mh/s for 4gb vs. 24Mh/s for 8gb version. Still, i would vote for 4gb reference model.
Difference of hashrate was bigger than the difference of price, so I finally opted for the 8 Gb version. Thank you for your help
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By the way, changing BIOS, if easy enough, is not a no-go option for me.