Difficult? Expensive? My friend, you would need a whole new system board and a whole new CPU. You'd essentially need to replace the whole thing inside, which would run you a lot of money, since you can't just pick up Macintosh components at the store. You could try hunting online, but why go through all the trouble? It's a laptop, not a desktop. Laptops have little to no upgrade capabilities. And even if you managed to get all of the parts you may need to retrofit the PowerBook with Intel internals, you'd have to modify the shell. Last I checked, the MacBook Pro has different dimensions than a PowerBook. The board might be too long for the PowerBook, which makes your goal harder to accomplish. I don't think you want to go this route. You would be better off getting a new Intel Mac.
If you really want a newer, Intel-based Mac, wait a few weeks and buy the new MacBook Pro when it comes out. According to the rumor mill, there will be a design overhaul that will distinguish it from the current aluminum-clad MacBooks and the older PowerBooks, alongside other good updates. I would seriously consider waiting until Apple refreshes all of the notebooks. Otherwise, you may end up kicking yourself later. If you need a replacement now, then go ahead and buy one; they are great machines, and you won't be disappointed. The MBP have 2.4-2.6Ghz of processing power and all of them have a 2GB of memory standard, with a 4GB RAM ceiling. The low-end MBP has a 256MB graphics card while the more powerful models have twice that. Any notebook you get will be quite sufficient for recording.
10.4.3? You should upgrade to 10.4.11, the latest, and last, version of Tiger. You could upgrade to Leopard if you choose to, but performance may be lacking on it. Also, if the PowerBook is in good condition, don't take it apart. I would choose to retire it to casual use or find a different purpose for it, once it is replaced, but it's up to you. Many Macs last a long time, and you may find that you will want to keep it around a little longer, should it be needed.
Really, consider a new MacBook Pro if you think you need more power. The Core 2 Duo chips blow the aging PowerPC chips out of the water, hands down. If you can, visit an Apple Store and try one out. You'll see the difference immediately, I'd think. But, if you want, you could upgrade the memory in your PowerBook to 2GB to get the most out of it for a little longer. According to this Apple Support page, it can accept that much. That should help. Well, I hope this info can help you out. Good luck, and let us know how you get on.
I have a PPC G4 15" powerbook that I purchased about two weeks prior to the Intel Dual Core's being released a couple years ago (1.67 GHZ, 512MB DDR2 SDRAM, OSX 10.4.3). How difficult (and/or expensive) would it be to gut it and upgrade my shell with a new, dual core processor and in the process greatly increase the performance and ram (I use it exclusively for recording music).
Thanks in advance for sharing your expertise...