1) You want a new machine.
2) You are cross-platform
3) You have a scalable twoer

I don't know how old your iBook is... but since you already have a tower that you indicate is continuing to work just fine, then I would suggest keeping it and getting a MacBook Pro.

Basically, merely upgrading the current portable environment.

Replacing the current portability with a tower eliminates your portability. I don't *think* you want to do that. When the existing tower is getting long in tooth, then consider replacing it with a tower... By the way, it is OK to be in a multi-OS environment. The diversity keeps you operational.

BTW... Congratulations!