This game isnt graphically intense so I believe you would see a better benefit from upgrading the CPU. Infact - my other half has only one step up the GPU that I have but has a 4ghz 8core CPU compared to my 3ghz 4core and it STOMPS on my performance. Also HDD speed counts alot for loading and instances/quests to update hense my upgrade to a SSD when I get my pay.

edit: this of course is all tested and written pre-directX upgrade that SE are planning on.