Hehe.... okay....
Where to start.
Amiga 2000/2500 are very similiar machines (2500 was a later edition with an accelerator board 020 or 030 iirc with a few megs of RAM, and i think ECS). They both use Zorro 2 as their main system bus, they are from the late eighties and thusly predate PCI. ISA slots exist on them but for all practical purposes (except maybe case fans, or if you like 386s) are useless. They can have accelerator boards shoved in them (anywhere from 020/14mhz to 060/50mhz) with ram and hard drive controllers, which make them speed up quite a bit. Most graphics cards available for them are Zorro 2, be Very Careful when buyign a gfx board to buy oen that is CyberGFX or Picasso 96 compatible(standard RTG software), others will have limited to no software support. There are a range of 16bit soundcards available also. The Delfina seems to be choice as it has a built in DSP (can play MPeg audio with almost no cpu strain) MAS devices also process audio with a DSP, and plug into the parallel port. Few 24bit gfx boards use the video slot, but a few do, also, genlocks and internal scandoublers tend to occupy it, lastly, the Video Toaster, an old but very capable linear video editing board can go in that video slot. There are of course USB, serial/parallel, video capture, ethernet, and just about every other concieved of expansion available for the Zorro 2 bus. The 2000 may Not have PCI boards, or PPC accelerators, but within those constraints can be made into a pretty respectable machine, capable of running OS3.9 with broadband internet and 24bit graphics.
I should repeat: the 2000 was built in the late 80s, it is not a modern machine in many respects, and can only be upgraded with a specialised range of hardware.
The 3000/4000/1200 can get PPC accelerators, and PCI boards . If you really wanted PCI, don't get a 2000 (which includes the 2500 and 1500).
PS: OS4 needs PPC, only prerelease is out on only AmigaOne boards right now.