Well it somewhat depends on what you want to do,

If you only want to do strcutured pvp and pve then a lowish end pc (gtx460, 4gb ram, AMD IIx2 250) is enough for medium details.

If you want to do wvw then you absolutely need a good processor (wvw is extremely demanding on your cpu),
For low details an AMD FX 4170 is enough
for medium details somewhere around the Intel i5 2500k
For max details you really need a current higher end cpu.

If you have a really good graphics card you may use a cpu slightly under the specs but not by much.