It's because of W7. It still does some things after boot, but eventually releases 200-250MB more of RAM and settles at 900-950MB (from 6GB).
I don't mean this as a flaw of W7. I'm using W7x64 on my old Opty170/NF4 too and it works great. It drops from 600-700MB to 200-300MB when LinX is ran. I run and stop LinX 2-3 times just for Win to release memory and then run it with desired RAM amount.
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