Hia, welcome to the forums
Gigabyte has a qualified memory list for your mother board at
http://download.gigabyte.eu/FileList/Memory/mb_memory_ga-ma770-ud3_v.2.0.pdfThe list shows the memory sticks that are guaranteed to work with your motherboard.
Looking at that list, I see that you could use, for example, 2 sticks of 2GB CORSAIR CM2X2048-8500C5D which run at 1066 MHz.
Obviously there are plenty of memory modules which aren't on that list.
Personally, if I were you, I'd buy another couple of sticks of what you already have to make it up to 8Gb. Of course the big question there is what is your operating system? 32 bit OS's only support a max of around 3.5Gb memory, so it's pointless having more than 4Gb of RAM. So unless you have a 64 bit operating system, you're stuck with 4Gb.
If you do buy new memory sticks (especially if second hand) make sure you test them immediately after installation using Memtest86
http://www.memtest86.com/ Before you boot into your operating System.
Ali.