To answer your questions to the best of my knowledge, correct me if I'm wrong:
-If I now own a normal sim card I need to change to a 3G SIM card?
Depends on your phone and the speed you hope to surf. To enjoy 3G, you need a 3G phone and 3G SIM (normal or micro size depends on your phone).
You can also choose to use GPRS/EDGE using 2G, but will be slow. But probably OK if you just use it only occasionally for some quick info checks.
Then again 3G is everywhere now, so I would recommend you choose 3G if possible.
Not sure about the exact differences. But they are radically different in term of design within to the engineers.
The 3G UMTS system added
increased security and improved address book. Visually to a layman like me, a 2G SIM and 3G SIM of same size looks physically identical, and I can't tell the difference. However, obviously using a 2G SIM in a 3G phone will not connect you to the 3G network.
-Which should I buy if I want to purchase prepaid data plans such as those shown (the 200MB for $10 one) : ("Most Affordable Data Plans" Section) -- the "Benefits | Singtel" page, the first result when searching "Benefits mobile data plans singtel prepaid"
It depends on your usage pattern I believe. Do you use data intensively over a long or short period, or you seldom use data at all except on some ad hoc basis?
If you mainly use SMS and voice calls, you can make do without a data plan and still be charged according to data volume transferred. The rates can be 2.7 cents or 5.4 cents per 10kb transferred.
However, if you are planning to surf Web, watch video, run mobile apps, do frequent social networking, message using WhatsApp, WeChat and LINE, data plans gives you better rates and value-for-money i.e. more data at lower costs.
I believe any excess data transferred that exceeds the max quota, or data transferred after expiry of your subscribed data plan period will get charged as per normal (per 10kb block).
I think what I need to do is buy a 3G SIM Card (but I don't know which one), and then use the phone to buy a prepaid plan (through calling *363 to subscribe -- but how does that work?)
I believe if you're looking at SingTel, there's only the Hi!Card?
Never tried this before, but I'm making a guess here.
With the Hi!Card inserted, you should be connected to SingTel 3G network. Dial *363 and call like it's a normal phone number. I believe your phone will then provide you with instructions once connected. They can be either in the form of audio interactive voice response system (press number and # hash key) or visual text menus on your phone screen (press certain number for certain option). Once the data plan is selected and confirmed, SingTel will deduct the data plan payment from your Hi!Card balance.