I'd rather buy a second hand Mac Mini or PowerBook. I reckon most others who like OS4 but can't afford an X5000 would too.
Those old Mac computers are very outdated hardware. It cannot match A1222 in any aspect if we go trought hardware specification.
For example:
FrontSideBus:
Mac Mini: 167 MHz
A1222: 400 MHz
Memory subsystem:
Mac Mini: Max 1G 333 MHz DDR SDRAM
A1222: Max 8GB DDR3 800 MHz
Expansion GFX card:
Mac Mini: old limited AGP card
A1222: modern PCI Express 4x with any RadeON cards that has a driver support for AOS4
Disk interface:
Mac Mini: Ultra ATA/100
A1222: SATA 2.6 compliant controller
Network:
Mac Mini: 1x 10/100 Base-T
A1222: 2x 1000 Base-T
Overall IMHO CPU is better in Mac Mini at the moment since it has Altivec, bigger caches and can range to 1.5 GHz. But CPU in A1222 is a dual core issue in contrast to Mac Mini which has only single core. When AOS4 start to support second core A1222 with P1022 CPU will outperform PowerPC G4 (7447A) CPU in Mac Mini.
However it is really questionable if Mac Mini CPU can be faster than A1222 right now since it has much slower memory, system bus, disc subsystem and in the end bus to graphic card. IMHO it cant right now.
Also you can have only 64 MB GFX RAM on Mac Mini. You can install 2 GB GDDR5 card in A1222. I have Asus R7 250 with 2 GB GDDR5 in my A1222 and it works great.
-Dooz