Hi kingex and welcome...
So memtest86+ v4.0 reports 4GB as well ? You need to follow outline in first post of this thread at Asus X58 + 6/12GB memory overclocking - detecting less memory than installed
- adjusting just dram memory voltage isn't enough - the key is QPI/DRAM and DRAM memory voltage balance. The first thing I'd do is test each modules individually as outlined in first post mark each module with number 1, 2 and 3 with masking tape as well as play musical chairs with each module to try them in different combinations in slots 2+4+6
Would be worth starting your own new thread in Asus intel board/cpu forum http://i4memory.com/f54/
outlining full hardware specs, bios settings used/tried and the resulting outcomes so far.