badge :
Radeon 3870>8800GTS 640 generally speaking.
http://www23.tomshardware.com/graphics_2007.html?modelx=33&model1=1060&model2=707&chart=315
Sorry but over all, the GTS still comes out on top.
As far as SLI/Crossfire goes....
I would prefer the Crossfire setup.
I believe that the GTS SLI and 3870 CF will perform similarly but I have a dislike for nVidia's motherboards.
Before you replace anything, why don't you try to overclock your GTS?
I am running a GTS 640 on a 22" also and can personally attest to the performance gains to be had.
If you still want to spend some cash, wait till the 4xxx series comes out before you make a jump.
Only another month or two left so might as well.