With computers, the longer you wait the better--but you give up HAVING and USING the new machine, so you can't wait forever. It's a personal call.
Ideally you'd wait (6 months?) for an Intel-based Mac laptop that would almost certainly be able to run Windows at excellent speed, one way or another.
Until then, a PowerBook G4 is the best laptop on the planet (I won't claim the fastest) and new ones MIGHT be coming this month at Apple Expo (or after--if they have been delayed as some speculate).
You could also make do with a cheaper iBook, saving up for something faster after the Intel change.
As for Virtual PC--yes, it's a slow PC (at least until an Intel Mac version comes out, which I expect next year) but it's still a product I really like. Why?
* Both Mac and Windows without carrying two laptops. Convenience is worth the loss of speed.
* Easy data sharing between apps on both sides. No network needed, just drag and drop!
* There's something cool about having apps from two different OS's share the screen
That said, you may find you don't need VPC at all--especially if you keep your old Windows machine around "just in case."
I thought I would need a Windows PC and couldn't do everything on Mac. First I bought a new Windows PC, and then I bought VPC... and now it turns out I don't really need either. Mac apps do all that I need, and exchange data with my Windows clients just fine.
Re screens: I use those same apps you do, every day, and I have a 15" PB. A 17" would be an even better screen... but in fact, I wish I had the 12"!
Because the 12" is just SO much more portable. Then, at home, I'd have a 20" or 23" Cinema Display (or Dell flat display--some excellent deals can be found on their 20" widescreen). Best of both worlds.
That's what I'd do--get a 12" (if portability is key) or 15", PLUS a second display for home base.
Good luck.