As I understand it, GW2 is more of a CPU-intensive game than a GPU intensive game. You seem to have a decent CPU and the 9400 I believe meets the minimum requirements, so you should be okay. However, I would recommend getting a better GPU if you want to have decent settings and a decent FPS. Best of luck though, good luck with your purchase.
P.S. I used to "game" on a PNY 9400 with a single-core Pentium 4 for almost 3 years. One day when I was BARELY hitting 35 frames on Minecraft and was ready to throw my computer out the window, I decided I needed to build my own system. Now I have a system with a Radeon 6950 and an AMD Phenom II 965. I had good times with that card though, and its still in my family's computer to this day.