You can probably just plug your PCI cards into the PCI-X slots. PCI-X is backwards compatible with standard PCI. The only issue you MAY run into is if you have an old 5-volt PCI card as it will not fit into the 3.3 volt only PCI-X slot. However, most PCI cards made in the past 10+ years are "universal" cards with three notches and can run on either 5 volts or 3.3 volts.
Have a look here:
Your cards are most likely the universal cards shown on the bottom left image. PCI-X slots are the 3.3 volt 64 bit slots on the right hand side. They will fit and most PCI cards will play happily in a PCI-X slot. I have a bit of server gear with PCI-X slots and regular PCI cards plugged into them and they work just fine.