16x AA at Very High? 1920x1080? Thats kinda extreme for even the latest cards, though depends on what FPS you're looking for. my 5970 overclocked can do 4x around 60-70 FPS at 1920x1080. for Nvidia I sappose a 570 or better would do the job

Agree with Hank. About the only that makes any sense is going with a GTX570.

Yes, that's a very extreme setting. Latest video cards like the GTX 570 or HD6970 maybe able to handle such stress. But are you sure you want to do this? The new video cards are quite expensive.

More ram is always good. With your motherboard you can add another 4gb if you want. You also have the option of buying a ssd, or another hard drive.

You can even add 12 GB of RAM, but extra RAM that is not used doesn't make any difference for performance. Keep an eye on Physical Memory in Taskmanager to see if you're running low. Keep it running in the background while gaming, it makes graphs.

Or get an SSD if you don't already have one. A fast SSD (not the cheap ones) will make the OS, programs and games load up faster.

Since you can already play your games on highest settings your money would be wasted on a new graphics card I'm afraid.

There's really not much you're going to see out of this. I would recommend overclocking the video card more than anything else. Maybe throwing 2-4 more GB of RAM for overhead, but even then you're not using that extra memory most of hte time.

Really the best upgrade I can see is to go to a 24"+ monitor.

Also, overclocking your CPU will still help you because very few games are multithreaded right now. Hence your extra cores are sitting there doing nothing while 1-2 cores do all the work.

Might as well go for 3 cores instead of 6 cores for a CPU.

Yeap, appaerntly the overhead stress is going to be on the Video Card.
Heck at this video setting you are asking to go SLI for sure.
Are you ready to shell out around $600plus?

Will it be worth it for you to play Crysis at this level for this much money?
Think about it.