Welcome aboard. Hopefully we'll be able to assist you with your concerns today.
Here's a link to where you'll find the best help with your phone: Samsung Galaxy Prevail - Android Forums
Also, you should move all the large installed applications to the phone's sd card. This should get rid of the annoying popup message.
A next thing to do, is to replaced the sd card you now have with a bigger one. You can buy one off EBay or Amazon for a reasonable price.
I hope you'll find the information on here useful. Give us a shout if you should need further assistance.