Will depend upon what is between the loft and the downstairs, 2 Ceilings/floors, steel beams, brick etc as to how fast and effective a wireless setup would work.
If it is perfect than you will have the same speeds as wired network [for the internet] and any inference will hinder speed on the wireless.
512k is quick for gaming.
Only way to know is to try it.
But a wired network will give you faster speeds in the home network. The intenret will be the same @ 512K.