On my PEG2 it makes no difference which PCI-Slot I use. For PEG1 I cannot speak. But its very new for me, that PEG1 should have PCI-X!!
I think all PEGs only have normal 3,3V-PCI-slots. Please pay attention that your card is no 5V-only-coded - it will not fit or damage system!
Only 3.3V or 3.3V&5V-dualcoded cards are useable.
Please refer to Pegasos manual!

I also don't know what SmartFirmware version PEG1 has - I hope it's similar to PEG2, otherwise booting won't work.