Heh... I had the same concerns coming from OS 9 myself, but I switched to Mac OS X 10.0 from OS 9, and boy was I disappointed with the speed... at first.
I soon discovered how to REALLY multitask with OS X -- OS 9's multitasking is a joke compared to OS X, and that alone increased my productivity ten-fold. Sure, menus and what-not were slower, but they've progressively gotten faster with each new release of OS X, and I'm happy to pay $130 a year to have the latest and greatest.
Give OS X a whirl -- do several things at once. Open five programs at the same time. Switch to Safari and surf the web while iMovie/Final Cut is rendering. The time saved being able to do multiple things at once without noticeable slowdown is amazing. I think you'll find that alone makes up for the menus and winows being slower than OS 9. It's a different way to compute altogether -- OS X isn't an extension of OS 9, it's brand-new and takes a while to get used to, but I think you'll soon be finding yourself wondering how you ever got by with OS 9 alone.
Good luck, and happy computing!