"I have just updated the RAM by 512mb this week, so should I take that out and try booting with original 256 stick in?"
Get some contact cleaner and clean the contacts on the RAM and the socket. Get some Dust-Off compressed air/gas and blow the socket clean after. Try the RAM again. If that don't work try the original 256 stick. Is your RAM the correct speed for your mobo?
Never heard of Medion before, but Gigabyte make good mobos.