It appears that the board does support 8gb.
Please review this document: http://downloadmirror.intel.com/15992/eng/DG45FC_SpecUpdate07.pdf
specifically section 1.5 of page 9, and table 1 on page 8.
The above document updates http://downloadmirror.intel.com/15992/eng/DG45FC_TechProdSpec.pdf
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