As you can see from my sig, my system is similar to yours apart from the graphics card, which I only changed from an ATI5770 a couple of weeks ago.
With the 5770, I could run on overall Medium settings, with shadows, grass and roads switched on, no AA, no epi, noo ssao, no v-sync. With these settings, the game was playable, with an average fps in Black Death track of 20fps (which of course is designed to test limits, with all the smoke and other effects).
When I upgraded the card to the one in the sig, (1280mb VRAM), with game maxed out and everything on except epi filter, the game runs as smooth as silk, with Black Death average of 30fps, the max being limited to 60fps by the v-sync. With this off, max frames go up to 80.
So if you can afford it ok, go for a better Graphics card is my opinion, but with the problems with crossfire and sli, a single core one. On the other hand, if the promised new graphics engine arrives soon, as this might resolve the crossfire / sli issues, maybe you could buy another 57** series and have it in crossfire, as long as you have the expansion slots on your motherboard. This would be cheaper of course, but given my experience so far, if you bought one like mine, you wouldn't be disappointed.
You don't mention your power supply, but I also had to upgrade this from the original Dell p/s in order to provide the card with enough juice.
Hope this helps.
