This kind of depends on your budget and what spec PC you have already.
I am able to play all the games you mention just fine with details on high using a single 5870 2GB card.
One thing to know and look out for when running multi screen gaming is VRAM on the gfx cards.
As you bump up the surround resolution, you require more and more VRAM otherwise you start getting major performance hits.
Although adding another GTX285 would be the cheapest option, using a 2nd hand card as they are no longer sold in stores.
However of cause the have limited VRAM, so you may find that you wont get all your games running in high detail with high fps.
The 460's with 2gb VRAM are very good performers as Paradigm Shifter has kindly
benchmarked the difference between 460's 1GB and 460's with 2GB. You will notice that when using Anti Alising at high resolutions with 1GB cards the FPS takes a great hit in comparison to a 2GB card.
Now the 5870 is a great performing card, and is a lot more flexible with what monitor configurations you can use. Again this card comes in 1GB and 2GB versions.
Basicly if you wish to play games with Anti-Alising switched on, then I would advise using a 1GB+ VRAM card (idealy 2GB).
I hope this can help you make an informed decision.
Personally I would go for either 2x 460's with 2GB VRAM each, or as I have now the 5870 with 2gb VRAM.