G'day and welcome to the forums.

If the G4 is your choice, go for the Mirror Disk Door 1.25Ghz single processor model. The 867 dual is a 133MHZ system bus speed, and the 1.25 runs at 167MHz and uses faster memory. Leopard will run very nicely, but you will need more than 512MG memory to be happy. 1GB m,inimum and suggest 2GB m akes for a very nice machine.

It will always be faster than the 867 MHz, which despite the dual processors, will only ever work at that speed, and not as some folk think twice the 867MHz.

Here are the 1.25 specs:-

Power Macintosh G4 1.25 (MDD 2003) Specs (Mirrored Drive Doors 2003, M9145LL/A, M8570, PowerMac3,6) @ EveryMac.com