How much VRAM does the card have? AA efficiency for the card is hugely dependent on VRAM, if it's too small than you'll take a huge performance impact, especially at 8x. Why don't you increase graphics quality instead of increasing AA?
You have increased your resolution by 65%. that is the card is now filling a space 65% bigger than it was before. Its about right for that card. Dont know what your CPU is but i had a system with a 4770 and a 1440x900 screen and your numbers at that res and settings are very close to what i was getting. I used 4x AA and had some of the settings shadows for one turned down a bit.
you will just have to play with it and see where your sweet spot is.