I'm always on IRC and really no need more buffer lines. Also my computer is slow, I can feel it when switching channels, - minimum 500 lines is too more.

Maybe can set this to 100?

This is already present: Option > Other > Window buffer

As Voglea stated, and I have confirmed on 3 different systems (Windows XP SP3, Vista, and Windows 7), the setting you reference won't go below 500.

While you can enter a lower number, as soon as the mirc.ini file is updated (I used /flashini to save time), this setting changes to the 500 count.

Ah, the wonky sentence threw me off. After re-reading the initial post I see the point.
I stand corrected.