Check out this
review of the '04 KG486 - the reviewer seemed quite impressed.
The 381 has been upgraded - now the 481SL - it is carbon tubes on alloy lugs and is lighter than the 486 (2.9 lbs). Laurent Jalabert rode a 381 to a King of the Mountains victory in the '02 Tour de France. It requires a 25-mm diameter seatpost, but Look supplies one of their carbon Ergopost 2 seatposts with this frame.
The 486 is a one-piece carbon design - it is a little beefier and stiffer than the 481 and has better aerodynamics. At 3.4 lbs it's not the lightest frame around but it's still no tank. Seatpost diameter on this frame is 27.2-mm so your options are not limited. Oscar Sevilla and Christoph Moreau rode this frame in last year's Tour de France, and this year the entire Kelme and Credit Agricole teams will be riding them. I think somebody also got 2nd place in the world TT championships last year on a 486.
Both frames are high-quality products with flawless attention to detail - either would be a primo bike that will make your ride mates drool. Which one is better for you I think mostly depends on taste (lugged versus monocoque, looks, etc.). The 481 has a slightly more aggressive geometry than the 486 - I would think the 481 is better for crits or shorter, hilly road races while the 486 is better for longer road races and time trialing.