Is there any point in me bringing along my standard European (Belgian) mobile phone (Nokia) to South America & buying a pre-pay-card to use overthere, if I need a phone?
I'm thinking calling a hostel to check if there's room or texting ppl to get together or something. Basicly the stuff that can't wait to be handled by email...
Another question is, if I would buy a pre-pay-card in one country, can I also use it in another country?

I'm not really a specialist, but I think you should check if your Belgian phone is dual band or tri band. Some phones in Europe are "only" dual band, and that means they don't work in Northern America where you need a tri band phone. However, I am not too sure for South America!?
South America's an interesting mix of standards. Some countries follow the European network standards, others the US. What you'll need really depends on which countries you're visiting. More on that in my FAQ under OnTheRoad/MobilePhones, link below.
By pre-paid card, you mean a SIM card? If so, unfortunately most local pre-paid SIM cards only work in the country of purchase, but it's worth checking with the specific network operator, occasionally roaming (the ability to use the card abroad) is an option.
If you're planning to use a local SIM, make sure your handset's not network locked. If it is compatible, and works with the network types used in the countries you're planning to visit, it might be worth getting it unlocked at home first. If you've been on contract for a while, they may give you the unlock code. Failing that most independent phone shops can help you out, it'll probably cost around €10 to get it unlocked.
The cheapest way to make international calls without a PC at both ends is an International Calling Card. Check out the options when you get over there, they're usually country specific. More on those in the FAQ under OnTheRoad/InternationalCalls:

I was maybe going to buy a local pre-pay-sim-card like the one I use here in Belgium, if that's possible. I'm guessing it might be usefull if I'm staying in one country for 4-8 weeks before travelling to the next?
Depends on whether you're planning to make/receive many calls. Roaming isn't cheap, in fact that's an understatement, but it is cheaper than buying a new SIM every time if you don't use it ;)
Remember that if you're using a local SIM, your phone number changes.
I tend to roam, but not make/receive calls. I use SMS, and if I need to call home I use an International Calling Card on a local phone.