If I can afford one good cable, which component?

Follow the musics signal>>Source>Pre>Amp>Speaker. If you have not addressed power from the wall and conditioning, you could also start there.>>Outlet>Power Cable>Conditioner or Regenerator> Power Cable to Source etc.

Tom6897 said it all

The previous comments do not take into consideration the infinite variables of each setup.
It may be the easiest way to decide, but not the most accurate.
Fiinding the magic right part to try out upgrading cables/Ac stuff?
Don't know. (since it is not my setup)

If I had to start somewhere helping you, i would say replace the A/Cwall outlet with a (slightly better than average) Pass & Seymour 20 amp heavy duty outlet from Menards $3 or so. Avoid a Levinson (crap) from Builders Squuare

Then buy a $70 used Adcom AC515 (or later 315)
to experiment with a bit.. A cheap power condtioner.

To blow some actual money: A Shunyata Venom 3 to check out on either the pre or amp
http://www.musicdirect.com/p-9670-shunyata-venom-3-power-cable.aspx
(I use Pangea myself, but would suggest the Venom to you)

The one cheap cable to play with
Blue jeans or Kimber PBJ
http://www.audioadvisor.com/prodinfo.asp?number=KKPB

In my experience, putting the best power cord on preamp or integrated is the way to go. Not on the source, not on the power amp. And the rest of the cables, yeah, follow the signal. However, as Elizabeth said, it might be different in different set-ups even if only to a certain degree. Current from the wall and regenerators/conditioners is another subject.