I would not put all my technological eggs in one basket.
You already have a very nice laptop, that many people wish they could afford.
You have your portable needs covered and your PowerBook is more than capapable of running Leopard when it comes out.
If you bought a Mac Book AND an iMac right now you'd have 2 machines
essentially running off the same C2D processor build.
What you don't have right now is dual boot capabilities, dual core processing with universal binary applicatons, storage, a large
display and Windows gaming capabilties which are much improved with
at least 256 MB VRAM.
There really won't be any major improvements to laptops until Intel releases the 45nm Penryn processor late next year.
A current C2D iMac is basically a super well equipped MacBook Pro
with a larger and faster HD, a great display and a better GPU.
You get the same technology for much less than comparably equipped MacBook Pro.
Since you already have a PowerBook, I'd just make sure that it's maxed out on RAM.
What you don't have is a powerful all purpose desktop and you'll clearly benefit from working on a larger display.
As difficult as it may be to wait, I'd just sit tight for another few months
till the Leopard machines make their debut.
If you must buy something now, I'd go for the 24" iMac, 2 GB RAM with 256MB VRAM
It's simply the best bang for the buck giving you everything that you're missing.
Just be aware that Quad Core desktop chips will be available in a matter of months.