minimum "/" partition size

[Default User]

Hi! 

I installed Ubuntu 7.04 on an older computer (AMD k6-2 400 mhz
processor , 512mb ram, 40gb ide hd).  The ASUS P5A main system board has
a bios from 1999. I works okay with Debian Sarge, but Ubuntu 7.04 would
not boot, gave grub error 18.  So I reinstalled, manually partitioned,
making the / partition a "logical" partition. 

After that it booted okay, but on startup gave a message saying the bios
was pre-2000, do acpi=force to enable, and would not power down the
computer on shutdown. After adding acpi=force to /boot/grub/menu.lst the
computer powers down, but still spews out an ugly group of unformatted,
cryptic error messages featuring terms like "ioctl" and "device" upon
shutdown.  

Maybe it has to do with the / partition (the only partition other than a
"primary" swap partition at the end of the drive) being a "logical"
partition.  So I am reinstalling (again), manually partitioning with a
separate "primary" / partition of 8gb or less. This is because I suspect
this is a problem of and older bios that doesn't work unless the /
partition is 8gb or smaller. 

But what size to make the / partition? Too small and it won't hold
everything it needs to hold over time after kernel upgrades, etc.  Too
big and it wastes disk space. Also, with a separate "primary" /
partition, how should the rest of the disk be partitioned? 

PS - I am not going to try to upgrade the bios. One mistake or hardware
error during the process, and the computer is just a big expensive
paperweight. It's just too risky. And I'm too poor to set it on the
curb.  

Any advice?