Hi Andy
I don't have a mac, but had a similar problem:
Computer was running, but system wasn't booting and no vga signal. :cry:
After removing my graphic-card, cleaning it and putting it again in the agp-slot
my computer works fine again.(until now
)
I dont know if this could help you,
because i am quite sure (?) the mac mini has an onboard graphic-card.
I wish you the best of luck
bye
olaf