I note that my task manaer usually at a minimum says using 252 MB of 1225 MB or something.
But if I'm at around 600 MB, the computer slows down bad.
So I have to be considerabely less than half of what I can use.
Usually when I open mIRC, it has less MB usage than IE, but can catch up.
And this could constantly be solved by closing finished p.m. windows and doing the /clear.