AH from my experience is way behind the curve on graphics card requirements for good framerates. Better cards certainly can help, especially with FSAA settings, but its nothing like a current top end hog like Oblivion. Get an SLI setup if you need it for other games, but not just for AH IMHO.
I get very good results on my ancient radeon 9800pro on my secondary system....which is certainly no slouch in its day, but isnt even on the charts these days, and honestly its not a huge jump to my main system which runs a 6800gt, sure I can set the ground detail a little farther off, but its just no huge change for 99% of gameplay.