testing an HD 4870 in my rig, only seeing 20-30 fps on Ultra high with no AA, get around 50-60 fps on very high with 2X AA , using the latest hotfix driver. Seems using quad core CPu's might be making a difference as another rig with an e2220 was getting laughable framerates with a 4870 until it was overclocked to over 3.3ghz. Sigh
Your problem is a LACK OF BIG RAM.....my 4850 1 gig allows me to play the game maxed+4 aa 1920x1200.