Okay so I am writing this message through my iMac via the "chicken of the VNC" client running on my Powerbook and it's pretty cool actually.
I'm running on a 1mbs wireless connection and its not too bad. There is stickiness, particularly when typing text (there's always an initial delay and then the text starts flowing) but its no worse than the Mcrosoft remote desktop I use to connect to my office windows pc.
I need to play around with this a bit more and in particular access the iMac when I'm in the office rather than downstairs! But so far so good and its free!
So what you do is download the OSXVNC software on to your iMac and fire it up. It starts a server session and shows you what IP address and host name its using and you can also enter a password.
On your PowerBook, download the Chicken of the VNC software and fire it up also. Enter the IP address or hostname of your iMac and any password you may have given and then press connect and you're away.
cheers
vanilla