Well, the pins are the same in all PCI-E x8 and x16 standards, and they are pretty much the same across the board, the only real difference is the bandwith of the bus, which even on 1.1 is not going to be saturated (there is still a lot of headroom in terms of speed between PCI and CPU even on 1.1 I mean, the bus is faster than the components at each end)
There's always a bit of risk involved when putting newer stuff in an older system. If it was me doing this, I'd just go for it and see how I got on since after reading up a bit I'm pretty confident it would work.
do let us know how it goes if you bite the bullet, im always interested to see how things like this pan out