The "Irongate" is another name for the 750/751 chipset. This is an early chipset for the Athlon and should use Socket A instead of Slot A. Perhaps this is what the guy means - that you can't hook it up with a Slot A processor. As far as I know, your 500 MHz is a Socket A CPU and should work out just fine, but please don't hold me responsible if it for some reason shouldn't work out...
Try a search over at
Google with some suitable phrases. That should give you some additional information...
/Tiny