Here is a quick overview of what we found:

Low-end: Less than Radeon 9600, GeForceFX 5900 - not playable, less than 5FPS
Mid-range: Around Radeon 9800, Radeon X700, GeForce 5950, GeForce 6600 - playable at 800x600 with AA turned off
High-end: Radeon X800, GeForce 6800 - playable at 1024x768 or 1280x1024, with AA and AF depending on the exact model of card
Bleeding-edge: GeForce 7800 - playable at 1600x1200 with full AF

Tough requirements? Sure. Worth it? Definitely. Quick note: there is no SLI support at the moment for Lost Coast, partly because we couldn't get the driver to recognise our batch script as the HL2.exe application. Expect this to be fixed soon. As for CrossFire, we'll tell you on Monday when we publish that article.