add RAM to desktop computer

I recently changed my windows 7 from 32 Bit to 64 and would like to add more RAM as well. Here is what I have now...
Gigabyte main board GA-78LMT-USB 3, AMD FX4130 quad core processor, 4 GB memory with 3.8 usable,1 TB Seagate HD. How much memory can I add to this?
Thanks for any help.