Basically i want to know if this ram is compatible with GA-890FXA-UD% motherboard. using a AMD 1055t CPU
Not specific to your CPU, it depends on your motherboard chipset.
As you can see on this page http://www.gigabyte.com/products/product-page.aspx?pid=3417#sp your motherboard supports:
So, you need to use 1.5V DDR3 memory running at 1066, 1333, or 1866 MHz.
Note 1 is just about 32-bit Windows' stupidity (basically, use 64-bit Windows if you want to install 4GB or more and actually use it all) and Note 2 says:
That makes it sound like you can only use 2 slots for 1866Mhz DDR3 memory.
Generally it's safe to mix RAM of different speeds as long as you run them all at the lower speed. Also, you can install faster RAM than your system supports as long as it uses the right voltage, it'll just run at a slower speed than it's capable of.