As unhappy as I am having to resort to a work around I have got IM running under VMware fusion on my iMac. For most part it works great, on occasions I get a slow down and I think this is a possible network issue rather than CPU or memory. I have Fusion running in "Unity Mode" this is where the applications within the windows environment give the impression of separating from the primary Windows display. I am able to easily drag photos from iPhoto straight in to letters in IM. For the uninitiated VMware Fusion is a program you buy that allows you to install other operating systems (Windows, Linux, etc.) on your PC/Mac. Once installed you can then install the other operating systems and run them while still running OSX. The newly installed OSs think that they own the hardware that they are running on, and are totally unaware they share the hardware. When you install the new OS you tell it how much CPU / Memory / disk space the new OS can loan from the Mac, most of this can be re-adjusted at a later time if need be. Lastly, Fusion is not any good for trying to run high graphical intensive Windows games, they simple will not work, nor would I recommend using it on an iMac with less then 2GB RAM. As an FYI I have attached a few screenshots and system stats.
CPU 2.8 Ghz Intel Core Duo
HD 500Gb (30 Gb reserved for Fusion)
Windows XP SP2
Mem 1Gb reserved
HD 30 Gb