Christina Aguilera to Charlie Sheen: Howdy Neighbor!

Christina Aguilera had better check twice when she borrows a cup of sugar from her new neighbor ... because she's moving into Charlie Sheen's hood.

We've learned Xtina just dropped a cool $10 mil for an 11,000-square-foot mega-mansion in Mulholland Estates, which is a stone's throw from Bev Hills.

She's actually significantly downsizing, having just sold her $13.5 million mansion previously owned by Ozzy and Sharon Osbourne.

The new house is awesome ... with 6 bedrooms, 8 bathrooms, library, bar, screening room, salt water pool, spa, gazebo, bathhouse ... the works.