I've recently built my computer for playing GW2 and I'll just post some general resources I found out that may help you out (if you haven't bought the parts already).
When checking price parts, you have to check out this website: http://pcpartpicker.com/
It will compare prices on multiple stores and post deals for you.
Although some good advices have been posted on this thread, I would also post a build evaluation at this website: http://www.reddit.com/r/buildapc
Very helpful and knowledgeable people there. You can also find some links on the right part of the page with resources that helped me quite a lot on choosing my parts. EDIT: and on my build