Sinclair QDOS which ran on the Sinclair QL did preemptive multitasking on a 128KB machine (buggy as heck when it came out but it did precede the Amiga by almost a year and half).
Yes, the Amiga had proper honest multitasking. A normal boot of an Amiga system would typically result in over twenty processes running in the background. In fact, its particular style of multitasking (static absolute priorities) was well-suited to real-time operation. Back in the days when CD writers could create coasters from buffer under-runs, I had more success writing CDs using my 25MHz Amiga than my 400MHz PC. Also, it was a microkernel system with things like device drivers and filesystems as separate processes, which had some fairly nifty consequences. For instance, it took Linux ages to lose its single kernel spinlock, which was a problem because of the huge amount of stuff done in kernel space. The thing the Amiga didn't have (mostly because of lack of hardware capability) was memory protection.
The Amiga 1000 first came out with 256KB of RAM. It didn't have much space left after the OS had booted up though. It was the Amiga 500 that came out a little later that had 512KB.
