2g and 3g mobile phone service

[Reece]

Hi, I have had a pre paid phone account with Savvytel here in Australia and recently found out that the old 2g Vodafone system is to be phased out in july 23rd, it then changes to 3g with Optus, people with accounts newer than 31st July 2009 have been sent a replacement sim but old customers get squat, i found this out as my daughter got he new sim recently, I emailed Savvytel for advice on what happens to my credit and my existing phone number, I haven't had one reply since the 7th with 4 emails sent, very poor! I later found this statement from a Savvytel rep in another forum:

With the new agreements we have with the carriers, If you are a customer who had joined us after 31 of july 2009 you will have to move to 3g network or you will loose your service. We have only sent the new sim cards to the customers who have joined us after this date. If you have received a sim card that means you have joined us after 31 of july and you you will need move to the 3g network.

My question is will my old Nokia 3315 accept new 3g sim cards, don't know what the difference between 2g and 3g is!