I am travelling from Australia to Greece for 5 weeks. Use my mobile or by a SIM card there?

Most of the calls will be within the country and to relative/friends. I have international roaming activated on my phone. The rate for calls within the country will $1.20/minute. That is on top of my $49 contract.

Should I buy a simcard over there? Will it work in my phone. I have a Nokia 6280, purchased around 18 months ago.

Does your mobile have a removable sim card? and is your phone "unlocked"? Or can you only use it with your carrier? If your phone is not unlocked, take it to the place where u got it, and have them do it for you. When u get to Greece, you can buy a sim card which gives you a Greek cellular number. It costs about 5 euros. You will also have to buy minutes.........but you won't be paying $1.20 per minute!! You will be paying Greek rates which are pretty reasonable, and best of all, incoming calls to your cell phone are FREE to you.

As Dvatwork says, who knows well, the best to do is buy a card here and use it for local calls to avoid being charged roaming money on your bill. They cost cheaper and as for incoming calls from Australia you can pass on the number and they can call you on the new one. That way they will not be charged roaming on their calls.
That card has a yearly life limit and if you come again you can use it again.

Since you mention the 49 cap I gather you are you with Vodafone?

If so, just grab a Vodafone simcard while you're over there. If your phone is network locked then that will kill two birds with one stone...keep ya phone and have a Greek number.

Otherwise, just buy a cheap phone over there (of which there are PLENTY!), keep your Aussie mobile roaming but kill your voicemail so you don't get hammered with international call charges when someone who doesn't know you're overseas and leaves you some long rambling message.

Note to dvatwork - I've only encountered incoming call charges in North America...Australia has not charged for incoming calls since the early inception of mobile phones

If your mobile is unlocked you ll have no problem using a Greek's networls card
6280 is a a triband phone so you wont have any problem about gsm networks either
Staying for 5 weeks in Greece,no i dont think that is so good using your Australian sim card
I suggest you to have a Greek prepaid number to use it for receiving calls from AUstralia
also i recomend you to buy call-cards ro make your calls from Greece to Australia (are very cheap ),you can use it by any desk ,card phone (i mean no mobile) just by pressing the codes before calling (the codes who are into the card)
all of the have english instructions