Hi,
Usually, aMSN would take 30 to 50MB of memory for most accounts.. it will of course be a bit more if you have a huge contact list... but it's always lower than what the official client would be using...
There was a memory leak that caused a huge amount of memory for the first login (then normal for subsequent launches of amsn with the same account) or when you changed your display picture. That bug was fixed, but I do not know in which version it was fixed... You could try the latest SVN version (
http://amsn-project.net/wiki/SVN) just in case..
About the CPU, it is usually 0% on idle, with small 1% peaks when someone connects/disconnects/etc... A huge CPU usage is not normal... (someone recently reported that removing the webcam or running the first step of audio/video assistant would lower the CPU back to 0%, but this is unconfirmed..)
I hope that answers your questions..