it doesn't really matter how fast the PCI solution is, all the interconnects are there, everything is talking to each other via hardware (no software needed), so it is only as fast as the slowest componant in the chain.
Theoretically speaking, PCI is a shared bus with 133Mb/s bandwidth.
PCI Express 1x is a dedicated link of 250Mb/s up and 250Mb/s down.
It should be possible to combine multiple adapters in multiple PCI slots to create an electrically compatable 1x,2x, or 4x PCI Express slot, but as these are all hanging off of the same 133Mb/s PCI bus, there isn't any gain in performance, just a gain in expense.
N.B.
A PCI Express Graphics card with a 16x connector will run in 1x mode if that is all it has connected, and (in theory) will be quite able to eat up all the bandwidth a PCI bus can trow at it even in 1x mode...
to fit an 16x card in a 1x slot, just take off the end of the slot allowing the card to fit "through" slot itself.
