Is it possible to have two Ethernet (cat5) cables between two PCs and
double the bandwidth between the two (assuming they were crossover
cables)? As a variant of that, would it be possible to have two
normal Ethernet cables going from one PC to a switch and two more
cables from that switch to a second PC, and double the bandwidth
between the two that way?

I don't know the right terminology. Is that what is called multipath?

Anyway, if it is possible, how is it set up? Does something go into
the Ethernet card configuration files to say they are special? Does
it require some special feature/driver in the kernel? Etc.

Thanks.

Channel booding is exacly what you are looking for. (Googling Beowulf project)

aiza, pardons for correcting your typo... its called channel bonding.

Its called channel bonding, Its come from Beowulf supercomputer project.
(I think search engine will help you much with that keyword)

Hope this useful
Aiza

I've done this with windows but never tried it in linux (where I work uses windows and I don't have the need for it). What I have used is Fast Ether-Channel or FEC. I think it requires a switch. It bonds the 2 NICs together and at 100 full duplex this gives 200 megs each way so a maximum of 400mb/s. I'll have to look into this some more and see if I can find anything about this in linux or about doing it directly with crossovers instead of having to configure the switch.