How much RAM is enough for a Minecraft server?

I've got a small minecraft server in my basement and I was hoping to expand to a hosting company because the basement PC wasn't working out. Because I need it cheap I chose for the 768MB Dirt package on MCProHosting.
That would be enough if we were to be running vanilla normal survival but we're playing on an amplified world and I don't think 768MB is enough.
Any responce is appreciated,
Thanks!
(PS: don't mind my spelling, I'm a Belgian 15 year old)

Since you have already purchased it your best option is to try it, test the performance and upgrade if need be.