its a nice idea, one major concern is the cost of the electricity
free is good and all, but your power isn't free
i see you are in B.C. , i used to live in Washington, so i assume your price for a kilowatt hour of juice is pretty low? in that case , this is sorta viable
but, one big problem with those older systems will be memory limited
an 8mb pc will barely run anything but DOS
and you may consider only choosing boxen with pci slots, ISA nics can be tricky in linux sometimes, if you restrict your farm to systems with pci slots, you can use all pci nics, as you said, not anymore cost than ISA these days
i would limit this venture to a minimum system config of
pentium 100
16mb ram
200mb hard drive
pci NIC
as far as floppy or CDROM, i wouldn't put a permanent one of EITHER on these boxen
i don't normally leave a CD or floppy drive in my servers/crunchers at home, i just plug in a CD drive long enough to load the OS , then shut it down and pull it out, one CD drive to cover multiple systems
you can do the same with a floppy drive, once the OS is on the hard drive, why leave the floppy attached?
if you are going with floppy , i would recommend using Debian linux, its easy to install a base system with between 2 and 17 floppy disks, if you have a LAN in your house and a newer PCI nic, you can load up the bare system with 2-6 disks and then complete the base install from ftp
worse case, you complete the base install with 11 additional disks
with this base installation, you can ftp your client software onto the pc and start crunching away
the projects that lend themselves to such slow/old boxen include (in my opinion):
RC5 - distributed.net
DF - distributedfolding.org
G@H - gah.stanford.edu
the first two have very small work units, so i think they would be better, i have run G@H on some pentium 100Mhz 16mb pc's at work and depending on the actual work unit size, they can take a week or more to complete a single unit
with RC5 and DF , the WU's are so small that you could complete much more #'s
good luck!