like a previous poster said, most likely, but...
It'll very likely be quite capable to handle multiple tracks, but to a limit. I don't know or use logic very at all, so this is postulation, but the new PB's are quite capable. RAM and quick HD's are ALWAYS performance boosters, and so the 5400 will defly help. As far as a laptop goes, I'm not sure you could do too much better. A dual G% like your buddy's will definately be able to though. Also, there is limited expandability on the PB''s. on the 15 and higher, you can add a PC card (?) for more input, but that's about it. On the G% tower, you could add several cards that let you have more higs speed input if so desired. This may be significant if you are creating wiht a band, or whatever. A USB keyboard or two is one thing, but with the number of ports availible, you can't have too much high speed input on a PB.
If you don't absolutely need the portability, I'd go with a G5. More input, power, RAM, and potential in the future. ALSO, importantly, you can add upto 15000 RPM drives and have multiple drives at much higher speeds than most firewire drives. Also, if you are using the FW as an input, you loose the potential for FW HDDs very quickly on a PB.
A PB is likely the best mobile, and teh G5 will almost 100% definately satisfy your audio needs for years.
the 1.67 is cheaper/Mhz if you don't add the superdrive, and 10% more proscessor is 10% more.
Per dollar (not sure at all) I think that you get a lot more power with the tower vs. the PB, even if you don't go for the 2.5