Mark -
Leopard's system requirements are 867 MHz or higher. Is your processor 866 or 867? If it won't take Leopard, it could be due to a lack of minimum RAM. It requires 512MB of RAM at minimum.
If that's not the answer, then you'd want to try to get a Tiger installation disc. Personally, that's the direction I'd go. Leopard on that machine may not be the best choice for performance. Granted, you'd be a little more "up to date" but the overall performance of the system would be mediocre at best.
I have one of those guys running Tiger for fun and it's pretty zippy.