My apologies for the delayed response - it's been a busy day.
First, regarding a new Mac. Thank you for the very clear description of your needs! I think you'll be extremely happy with a new Mac given the processor-heavy work that you do. Everything will run significantly faster and more smoothly!
Given your needs, I think the MacBook and MacBook Air lines will not meet your needs (MacBook has a small screen, Air has a slightly bigger screen but no built-in DVD drive). This leaves the MacBook Pro line, which is a great combination for you: fast, big screen (15 or 17 inch), DVD burner, etc. Some specifications that you'll want to consider when purchasing from the Apple store:
15" or 17" screen - your preference
At least 2 GB of memory, since you run a lot of memory-intensive applications
The GHz speed is not a critical specification - the difference between a 2.4 and a 2.54 GHz machine is small
The bigger the hard drive, the better. This will give you plenty of storage for your graphics and videos
You'll also want to order an adapter to connect your external monitor
I highly recommend AppleCare for laptops, as I've ended up using it for every laptop that I've owned. Laptops get a lot more abuse than a desktop machine since they're moved around.
So, those are my thoughts on a new MacBook Pro for you! Let me know if you have any more questions, and I'll be glad to try and help.
Regarding backup, you'll want to copy your entire Users folder to an external drive. This will cover your settings, Documents, Movies, etc. folders. If you have stored files anywhere else on the hard drive (outside of the Users folder), back those up too.
Many applications are self-contained and can be copied to a backup with no problem. However, given the complexity of Photoshop, you'll probably need the original install disks for the program. Do you have them?
Once you've copied all of your files off...
Let's do one more test in the 10.5 vein.
1) Put the 10.5 install DVD into the drive
2) Restart while holding down the C key. This will boot the computer off of the CD
3) Once the list of languages comes up, click the arrow button
4) Go to the Utilities menu and choose "Reset Password..."
5) Click your hard drive. There is a text field below ("Enter a new password XXXXX") where you can test your keyboard. Try the same keys you were having problems with.
6) If the keys now work, then it's a software problem and installing 10.5 will fix it. If the keys don't work, then it's a hardware problem and you'll need to send it off.
If it turns out to be a software problem, then here are instructions for the "Archive and Install" feature you want to use. This will preserve your files on the drive (but, you should still back up first, just in case).
You're doing a fabulous job of helping solve these problems! Thanks for hanging in with me through this lengthy process. I very much appreciate the tip - while not all of it goes to me, the majority of it does.
Let me know how things go, and if you have any more questions! Best,