Rock Band and Guitar Hero has been singled out as driving a boom in digital song sales.

According to Billboard and Reuters, in the first two months since the release of Rock Band, which is a joint venture between MTV Networks and Harmonix, "players have purchased and downloaded more than 2.5 million additional songs made available after the game's initial distribution".

Activision says that its Guitar Hero III games has seen sales of more than five million new songs via download since the company started to add downloadable content in early November.

The original versions of both games shipped with more than 50 licensed songs each - which were a mix of master recordings and covers.

Rock Band owners have since been able to buy new music every week as either singles or in three-pack bundles that can be added as new playable levels for between 99 cents and $5.50.

Guitar Hero III gamers also had the same options.

It seems gamers has been quick on the take-up.

MTVN Music Group/Logo/Films division president Van Toffler told Reuters: "With such a low installation base, we didn't think that there'd be 2 million songs sold in eight weeks".

"We live in a rough time around music where our audience struggles to pay $20 for a CD but don't hesitate to pay $50 for a game. The notion to pay 99 cents or $1.99 to have a song and repeatedly play with it apparently isn't a big hurdle."

But analysts are yet to determine whether the increasing sales of music for these two games can have a real impact on music sales for the artists whose music is being sung or played along to.

MTV will not release exact figures but did say that the Metallica three-pack of "Ride the Lightning," "Blackened" and "And Justice for All" is its best-selling Rock Band download.

Nielsen SoundScan data revealed that those songs did see an increase in digital downloads (not for the games but for purely listening to).

For a month after they were featured as a Rock band download, the songs enjoyed increases of 31%, 39% and 48%, respectively.

But Reuters adds that these increased sales were in the hundreds while the Rock Band downloads figured in to hundreds of thousands.

But nevertheless, studio executives see the games as offering marketing potential.

The game developers, however, have recognised the popularity of music downloads for games and MTVN told Reuters it already has plans to expand its outreach to artists, "creating additional game expansions - as both physical products and downloadable content - around specific music genres and even artists", the agency reports.

Toffler added: "We are talking to tons of bands, from indie to the most established ... to release not necessarily their entire catalog, but maybe some of their classic albums and do special packages around that".

And it may be that Guitar Hero and Rock Band are soon joined by a host of other games where gamers get to choose the soundtrack they play to.

Reuters points to titles including the "Madden" football series, the Tony Hawk skateboarding franchise and the "Grand Theft Auto" games and suggests that the option to replace your games'soundtrack every couple of months may soon be a reality.