Really it all depens on the native resolution of the new monitor. If it's.. say... 1680X1050, it probably won't be much of an issue, but if it's 2560X1600, that might be a problem.
I used to run AHII on a 7900GS pretty well at 1680X1050, even with AAX2 enabled... got a steady 50-60FPS (1024res@hires)... BUT that was before the updates.
I've since upgraded to a faster C2D and a 9800GTXv2