Seeing as GarageBand already gives you 64 tracks, and it's free - if you're reading about a 12-track limitation in LE, then it's *certain* to be in simulaneous record-enabling, as you suspect.
The machine will run Logic okay, but figure on an external firewire-drive.
The internal drive is a laptop disk and is comparatively slow.
Also, the iMac G5 has a 600 MHz system bus, so you're getting roughly 4x the data throughput when compared to the G4 boards.
And once you think about it that way, the iMac G5 is starting to look mighty fine.
(apart from the 2GB max for the G5, self-installable).
-s*