After Commodore went bust, essensially no new machines were designed and made (except the new ones that you're not interested in). Lots of add-on hardware and software has been developed though.
The last ever Kickstart ROM chip was 3.1, but after that two new OSes were released - OS3.5 and OS3.9. If you have Kickstart3.1 and the correct
system requirements, then you can install that.
You will need to upgrade a bit to reach that standard - ie add a CD-ROM drive and an accelerator. There are far more expansions availible though. Later accelerator cards came with PowerPC processors (the same CPU as Macs). You can get the CyberStorm cards for the A4000, or the Blizzard cards for the A1200. While AmigaOS3.9 or below doesn't utilitise these fully (the upcoming OS4 will), lots of multimedia software does.
PCI busboards are also availble - with one of these, you can add a selection of standard PC cards to the Amiga (eg Voodoo3 graphics card, SoundBlaster sound card, network card, TV card etc...) If you want to soup up your A1200 like this, you will need to put it into a tower case. With all of these expansions, you could have an OS4 ready Amiga which would easily give your PC a run for it's money - it would also cost
a lot.