first of all you need to ask about what budget to play with, also a mid end GPU will cost near to a I5 cpu + cheaper end mobo to go with it anyways, which will be a huge boost, the new haswell cpus have a half decent integrated gpu to start with, you can pick up a mid end gpu later
Video Card:Gigabyte Radeon HD 7950 3GB Video Card (£249.98 @ Amazon UK) Total: £249.98 (Prices include shipping, taxes, and discounts when available.) (Generated by PCPartPicker 2013-06-26 15:27 BST+0100)
When there are a lot of players on the map in WvWvW then I get some lag but I am playing on a slow ADSL connection and also at 1920 x 1080 resolution. I also get a bit of lag at the very start of the fire elemental. Not sure why. That's with the 7750.
I would replace your CPU with a better one that will fit in the MB if you can. You might be able to get a Phenom II X6 or something like that which will fit. This would more than double your CPU speed. Here is a list of CPU's supported by your MB:
My daughter is using this combination (with a 7750) and doesn't have any problems in GW2. She doesn't play WvW though.
That puts the total at £211.
You could then add the PSU and RAM from the post above and come in at £301. Not much of an expert regarding parts, can just tell you what is working for me. I am using the corsair PSU above but corsair value RAM. If at all possible you should try and get a better GFX card for the future.