This may have been posted before, but I think it is worth another mention.  It looks Microsoft has changed its mind and will now allow users who merely built their systems to get the free upgrade to Windows x64 as well. 
BETA NEWS reported yesterday that Microsoft has tweaked its upgrade programme and has now entered a world closer to reality.

According to the site, Microsoft will now give users an upgrade to the 64-bit version whether or not they bought or built their PC.

Formerly, the rule was that the programme only applied to PCs bought with an OEM copy of Winders.