Rock Band, the game released by MTV and Harmonix, has reached the six million downloads mark. Also, Harmonix has announced a major patch for this week and a full-featured, in-game music store. The store will have a "user-friendly interface to streamline and enhance the music shopping experience," and will let players "view, purchase, and sort all downloadable songs tracks by Artist, Song Title, Genre, Albums, Pack," with full album art, song previews, and instrument-by-instrument difficulty levels, the company said

Rock Band is a party game that was released bundled with controllers shaped like guitar, drum set and microphone. The full package costs $170 and it seems the price doesn’t represent a problem among the music-themed video games fans.

"By empowering players with the ability to customize their Rock Band library with a rapidly growing selection of rock artists and songs on a weekly basis and adding great new features such as the in-game music store, we are thrilled to give fans more ways to enhance and enjoy their Rock Band experience," Paul DeGooyer, senior vice president of DVD, Gaming, and Audio with MTV, said in a statement.

The game includes 58 playable songs such as the Rolling Stones' Gimme Shelter and Metallica's Enter Sandman and it was released for Xbox 360, PS2 and PS3. More tunes can be downloaded over the PlayStation Network and Xbox Live for prices varying from 99 cents to $2.99.

A Metallica pack featuring three of the mega-platinum band’s most electrifying tracks—“Ride The Lightning,” “Blackened” and “And Justice For All” — is constantly among the game’s top-selling music downloads, followed by three-song packs from The Police, Queens of the Stone Age, David Bowie and a collection of Black Sabbath covers.

Top-performing singles are Foreigner’s early ‘80s hit, “Juke Box Hero,” followed by a cover of Creedence Clearwater Revival’s late ‘60s smash, “Fortunate Son,” Weezer’s mid-90s single “Buddy Holly” and a cover of The Knack’s blockbuster hit, “My Sharona.”

Also MTV intends to use the downloadable content as a way to promote new songs and albums. Rock Band represents the first act of MTV’s plan to spend more than $500 million on video games over the next two years as Viacom’s company expands beyond its TV base.