Yeah, this is a problem with the Mac's memory architecture of yore that is about to rear its head again.
The original Macs had a memory address space of 24bits, which limited the machines to 16MB of memory. Half of that was reserved for expansion cards, ROM, etc. So you could only have 8MB tops.
This seemed like an acceptable, if kind of dumb decision back in the days of 128kB Macs, but was just plain dumb by the end of the 80's, if not before. So Apple finally utilized the entire 32bit address space of the 68K CPU, and wound up with a 4GB upper limit. (parts of which are still reserved for other purposes)
Obviously, you can see the problem: you can buy 4GB of RAM for somewhere in the neighborhood of $500. Apple needs to migrate to a 64bit address space, which might require new CPUs, or develop some sort of hack (which would suck, rather like the DOS 'extended memory' hack always did) to cope with ever-falling prices and ever-increasing densities.
Anyway, you want to either switch 32bit addressing on, in some Apple cdev that shipped with later versions of the OS, (although you cannot run 7.6 or above, IIRC) or the free Connectix cdev, MODE32, which does the same thing, and had a better reputation.
Incidentally, however, IIRC that the IIsi was still limited to 17MB tops, rather like the Classic (10MB) because of yet another dumb limitation in Apple's design, regardless of the 32bit issue. You might want to check -- I never heard of a IIsi with more memory in it, anyway.
[ 12-19-2001: Message edited by: cpt kangarooski ]