How easy hard is it to upgrade slower models with a bett GPU, memory , CPU, hard drive.
GPU cannot be upgraded (all minis have Radeon 9200). Graphics memory and CPU upgrade require change of the motherboard, however CPU can be overclocked. System memory is easy to upgrade, maximum is 1GB (with single DDR ram stick). Hard drive is easy to upgrade, the maximum is the largest 2.5" PATA drive.
See here for details:
Mac mini (PowerPC) Repair ManualOverclocking is covered by
this thread, especially
post #10.