Although how much memory is needed I'm still not sure - any ideas on this?)
That depends for 100% on what you're actually *doing*
The more sprites, backgrounds, etc. and the higher the bit depht (16bit or 32 bit?), the more memory you'll need.
Many small games will run just fine on a machine with 16MB, but once you use more graphics than will fit in that size of memory, you'll need to bump up the requirements to the next size, 32 MB. and so on.
But its pretty much just guesswork to find out how much memory you're using. Don't look at the actual JPG file size, but look at the number of bits necesary to contain the uncompressed image -- rounded up to the next power-of-2 boundaries.
Also keep in mind the memory required for the windows desktop, and the backbuffers.
but needless to say, the video memory requirements for 'pacman' aren't even in the same ballpark as those for 'doom 3', so without knowing what you're doing you can't give any relevant estimate.