Man I've been living in the country for too long :/
As for overclocking PCI cards - that depends on what you mean. Some PCI cards can be overclocked (e.g. graphics cards - but you'd be OCing the actual GPU on the board), and it's (on some motherboards) possible to give the PCI bus itself a small boost. However, OCing the bus won't help, since that only affects communication inside your machine; you would need a find to "overclock" the actual signal leaving the port, which, even if possible, wouldn't help you since the non-overclocked device at the other end of the cable would most likely not be able to correctly receive the signal.
tl;dr: can't be done. On the plus side, 1Gbps cards are insanely cheap these days (I've frequently seen them go for less than $10 around here), so that shouldn't be a problem.