im gonna get my 8700c unlocked...i will put a prepaid t-mobile sim in...its a sidekick pre-paid sim that means i used it in a sidekick nd has the internet...can i still get online with this pre-paid sim???

Very very doubtful.

You need to get the BB unlocked:
How do I unlock my BlackBerry? - BlackBerryFAQ

Pre-paid SIMs don't normally work with data, you will need a BlackBerry Data Plan, or the Feature Enabler with TMobile.
What do I need a Data Plan for? - BlackBerryFAQ
How to use BlackBerry on Generic Data Plan - BlackBerryFAQ

Good luck.

yea i guess your right well ill try and then if it doenst work ill just sell my sidekick 3 and blackberry 8700c for a curve or a pearl

You may be able to get some data on it with the proper TCP settings but results do vary widely. No guarantees.