The simple answer is this:
With A1200, fast ram will double the speed of most tasks. A cpu card with no fast ram will only speed up cpu-intensive tasks. A cpu card with fast ram on it will make the biggest speed improvement.
If you want to do things like play games, mess about with 2D paint and animation software, then just get a cheap RAM card and a compact flash card and stick whdload on it with a preconfigured Workbench environment like ClassicWB (use the version thats made for an A1200 with a fast ram only.)
