The first computer I owned was a Heathkit ET3400. This was a MC6800 based microprocessor training system I had to have for tech school (circa 1980). It came with one 256 byte bank of memory and had sockets for a second. The person I bought it from had installed the second bank. I ended up buying a third pair of chips and stacking them on top of one of the existing banks. Internally, the system decoded for four banks so i just had to run a wire to the enable lines. With the three banks, I had a whole 768 bytes of RAM.

The first PC class machine I owned was actually an office machine. I worked for DEC and they gave me a Rainbow-100 to use at home. The Rainbow had a Z80 and an 8088 and could run CPM, CPM-86 and MS-DOS programs. I think the Rainbow design was better in some aspects than the IBM PC but DEC made some critical mistakes like not giving the machine a bus that you could plug cards into. It had slots on the motherboard that would accept specific option cards. Since no one else but DEC could make cards for this system, it didn't quite catch on. I still have this machine in the basement (it wasn't worth much when I left DEC so they let me keep it) and it still works.

The first PC class machine I bought was a 20MHz 486-SX in June 1992. By November, the 100MB hard drive had a linux partition on it.