What is your budget? What is the computer not doing well right now that made you decide you needed an upgrade? It looks like your motherboard uses PCI-E, so you could upgrade to just about any current GPU you wanted. What CPU do you currently have? That will probably become the bottleneck if you go with a fast GPU.
I'm guessing you must be from Germany since I've never heard of that RAM, but a Google search brought up a bunch of German sites. You could try buying another 2GB kit that has the same specs as your current RAM, but you would be better off just replacing your current kit with a 2x2GB kit.