I think i am going to go for the 2.8
I registered just so I could reply. I've been lurking the forums for a while now.
I teach at a university in a lab with 8 Mac Pros and two G5 workstations. The G5's have 003 systems, the Pros all have Mbox Minis.
I wanted to mention that you will see a lot more of a performance impact from the amount of RAM than from the processor speed with the current configurations. 2.4 - 2.53 - 2.8, you are talking about a maximum of a 400mhz difference.
I find that in virtual instrument usage the G5s in the lab are way more usable as they were originally installed as Final Cut Pro workstations and as such were given more RAM. About double what the Mac Pros have. I'm not in the room right now, but I believe it is 2GB vs 1GB (I know, I know, but I didn't consult on the install, I just added the PT setups and hardware to teach my classes).
In Native Instruments (currently Komplete 4) we can get any two instruments humming along fine. On the Pros if we add a third the machine dies. We can get two or three more on the G5's. Opening Activity Monitor shows that a ton of VM paging is the issue.
Both sets of machines are 4 core, and honestly I don't know the speeds, but the Mac Pros, if configured correctly, should annihilate the G5's.
To answer your original question, I have the original Macbook Core Duo (not 2) with 2GB of RAM, and I'm pretty happy with Pro Tools 7.4 performance on it. I also use the Native Instruments Komplete 4 package and have had pretty good luck. I don't often use more than a couple of VIs at a time though as I'm doing Sound Design work, not so much music production. I was also able to generate some HD video, and do a small amount of post-production work in PT with the HD video on this Macbook. So either new machine should be totally fine for you in that aspect.
And it sounds like you've decided, but I did want to say, that in my opinion, for me, I could never consider a non-firewire equipped machine. Of course I have a 002 at home and a few firewire drives. To take that with a grain of salt.
