What graphics card are you using, and what kind of performance do you get?
I'm currently using a 5870 with 2x AA, 16x AF, and everything else set to ultra (except for water and shadows) and my fps isn't that great. I'm usually between 30-50fps. I've also tried lowering it to high, but that only gains a few frames.
Is it worth it to sell my 5870 and maybe go crossfire / sli?