Check out this blog
it might help.
For someone who doesn't fully understand that is a lot of information. One of the things that I have found can improve the performance greatly with the video card;
If you have a Good CPU, above 2.4Ghz decent bench mark and multi-core. Go for an nvidia card, the ones stated here, 560 Ti is a good solid card but your ATI card mentioned will also be OK. Will run the game at Max settings with a couple of control panel tweaks to help.
But if your CPU is, on the older side, then an nvidia card can help, reason is, offloading the physics processing from the CPU into the GPU will leave more room on the CPU for processing the TCP stack which can get hectic to say the least in high population areas. This is due to nvidias PhysX processing on the card. With a lot of the physics processing being optimised for this as well. AMD doesn't have this on the cards and subsequently means all of that processing is moved to CPU.
I only know this because I had a 2.4GHz Quad Core and a 570GTX OC this configuration enabled me to run max settings, except fleet would bog down and other areas would also cause problems, 8man and 16 mans ops, PvP battles, number of people, the amount of effects all take its toll. I switch the PhysX from Auto to GPU and it improved things a little
I upgraded to an i7 3.5GHz, and granted it meant I needed to upgrade a few more components as well, that move has given me more than perhaps upgrading to a 680 card on the old machine.