I think it was in v6.0 that mIRC started flushing incoming dcc get files to disk every 500k and then in (i think) v6.02 flush files was put into dcc/options allowing you to decide if you wanted to do that. My memorybanks may be flawed as to exact versions, but its in the versions.txt included with each release, or see all the versions.txts at once
here