8800GT > HD 3870
8800GT SLI < HD 3870 Crossfire
Simple maths. Crossfire scales better than SLI does. So go NVIDIA for single card or go ATI if you're doing a dual-card solution
ATI might become the better choice as a single card if ATI optimizes the drivers for it.
Some other info:
*8800GT uses up only one slot whereas the HD 3870 uses up two
*HD 3870 has been shown to have slightly better image quality, but runs slower
*HD 3870 is generally cheaper