I'd recommend using a multi-client program if you'd like to minimize memory usage and integrate all your contacts in one place.
I like Trillian Astra (but the WLM file transfers are very slow). Astra runs at about 11,000K on my machine. Digsby is also a good alternative, and I believe they have made significant memory improvements in recent months from what I've read. Take a look at the
interoperability matrix to see some other choices.
You'll do better with one multi-client running than several stand alone programs.