Check the power consumption of the card during load.
If it uses less than 75W you will be fine with your PCIe 1.0 slot.
If it uses more than that, you will need a 150W PCIe 2.0 slot.
Another option is to get a 9800GT with a 6 pin external power connector.
This one will work on PCIe 1.0 slot and the extra power will be supplied by te PSU.
I have a 8800GTS512 on P5B Deluxe and 25A @12V PSU and it works.