pci-x is backwards compat with pci, to a point. don't think of usb1.1 vs 2.0, think more of agp 1x vs 2x,4x,8x. the agp slot has under gone changes in the past. the 1x slot is backwards when compared to a newer slot, and therefore not compatible with newer mobos. but 2x thru 8x are compatible. the same goes for pci. the early pci cards had more pins on them, the newer cards have less, and pci-x is compatible with newer pci, but not older pci. so if your scsi pci card is older, it will not fit in a g5, but if it is newer, then it will. the problem these folks seem to have is that they want the old scsi, which noone makes newer cards for. if they want scsi, they will have to get a card the says it is g5 compatible ( if it is advertised as such, by the law it has to work, or they can be sued). the card will not be 50 pin, but scsi is backwards compatible. i have found all kinds of scsi adaptors on ebay. just get one that will change the new scsi pin out on the card to match the pin out on your device.
btw, at this point, a more important question comes up, is this old scsi device you are trying to use compatible with a g5 and 10.3?, better check that first, because if its not, then there is no point in getting a scsi card, no is there. i have found that over 90% of 50 pin scsi devices are not compatible with os x without some kind of beta hack or patch, which are generally not very stable.