I wondered if theres any way of putting a better
graphics card in my Mac Mini G4 (40Gb). And if I can
what cards can I fit in it and is there any guides?
Despite the suggestion that the G4 minis have an ATI 9200 graphics 'card' and that there should therefore be a way to either replace it or upgrade it, the graphics subsystem is not a separate component at all. It is, as the link Wayne provided clearly shows, integrated onto the motherboard of the mini and as such could only be upgraded by replacing the motherboard itself since it is barely possible that a domestic user or field engineer would have the skill to remove existing components, even if replacement with an alternative was technically possible.
Since there is no alternative motherboard, other than the so-called 'stealth' units which had the same graphics subsystem but with 64Mb instead of 32Mb dedicated video RAM (and I've yet to really note any significant impact that memory increase has given when comparing my two G4 minis) there is no alternative to the graphics 'card' you already have without replacing the entire system.