Following the release of the original Macintosh in 1984, Apple released the Macintosh 512K in 1985. The main difference between the two was the increase in RAM to 512 kB. The original Macintosh had 128 kB and started to be called the 128K after plans to release the 512K were revealed."
Any major computer upgrade generally requires a full software reinstall too. No so if you heed the advice in the guide to Upgrading a Motherboard without Reinstalling. PCSTATS Tips