A great deal depends upon which Amiga you're talking about!
68000 (16-bit data bus) Amigas (1000, 500, 600) perform noticeably better with TRUE (still 16-bit) fast ram. Adding a 32-bit processor (68020, 68030, 68040,68060) alone is a mere modest boost. A 32-bit accelerator with 32-bit fast ram gives a large boost in performance.
Everything depends on which Amiga and how far you ultimately intend to expand it.
That just about sums it up extremely well and has been my experience.
