I run 2 mIRCs. One with just a trivia/protection bot with a few additional scripts of mine (weather, dictionary, search). It runs at 2.8MB with a 5000 buffer. The second runs Invision, with around 239 open hash tables as well as a lot of other scripts of mine. It runs at about 10.6MB with a 3000 buffer.
The buffer is a big part of memory usage and unless you need it really high, 5k or less will reduce memory usage a lot. This is especially true if you're on many channels as your buffer usage is per channel.
Note that this is on Vista 32bit, but I think it was similar on XP.
