Apple has upgraded its Product Red iPod nano, adding an 8GB version to the originally announced 4GB model.

The company quietly added the second higher-capacity configuration of the product last night.

The 4GB version costs £129, as before, while the high-capacity model costs £169, effectively meaning you can have an 8GB iPod nano in your choice of black or red.

If you buy the red one, then a portion of the money spent will be directed to the Product Red-backed Global Fund that's dedicated to battling HIV/AIDS, Tuberculosis and Malaria.

The Global Fund to Fight AIDS, Tuberculosis and Malaria was established in 2002, with the support of the world’s leaders and UN Secretary General Kofi Annan.

On the launch of the Product Red iPod nano in mid-October, Apple CEO Steve Jobs said: “Now customers can buy the best music player in the world and do something to help the world at the same time. We’re honoured to work with Bono, (RED) and this team to contribute to an incredibly important initiative and help change the way people think about AIDS in Africa”.

Anecdotal reports claim that Apple has seen stronger-than-expected sales of the commercial fundraising product.

UPDATE: Apple vice president of worldwide iPod product marketing, Greg Joswiak, confirmed strong Product Red iPod sales today, saying: "Customer response to the iPod nano Product Red Special Edition has been off the charts. We’re thrilled to add a second model with 8GB of capacity, enough for 2,000 songs, so customers have yet another choice in supporting this important cause."